调试易

 function txtoldPwdBlur(obj)
        {
            var msg1=document.getElementById("span1");
            if(obj.value=="")
            {
                msg1.style.display="block";
                msg1.innerHTML="原密码不能为空";
                obj.focus();
            }
            else
            {
                msg1.style.display="none";
            }
        }
<input id="oldPwd" name="oldPwd" type="password" class="input" onblur="txtoldPwdBlur(this)" />

在IE7下有问题，但在fire fox里没有问题

function txtoldPwdBlur(obj)
        {
            var msg1=document.getElementById("span1");
            if(obj.value=="")
            {
                msg1.style.display="block";
                msg1.innerHTML="原密码不能为空";
                obj.focus();
            }
            else
            {
                msg1.style.display="none";
            }
        }
        
        function txtnewPwdBlur(obj)
        {
            var msg2=document.getElementById("span2");
            if(obj.value=="")
            {
               msg2.style.display="block";
               msg2.innerHTML="新密码不能为空";
               obj.focus();
            }
            else
            {
                msg2.style.display="none";
            }
        }
        
        function txtnewPwdAgainBlur(obj)
        {
            var msg3=document.getElementById("span3");
            var obj1=document.getElementById("txtnewPwd");
            if(obj.value=="")
            {
                msg3.style.display="block";
                msg3.innerHTML="重复新密码不能为空";
                obj.focus();
            }
            else if(obj.value!=obj1.value)
            {
                msg3.style.display="block";
                msg3.innerHTML="输入的两次新密码不相同";
                obj.focus();
            } 
            else
            {
                msg3.style.display="none";
            }
        }
<div id="zong">
                    <div class="title">
                        <img src="images/login_01.gif" /></div>
                    <div class="title_1">
                        <img src="images/login_06.gif" /></div>
                    <div class="content">
                        <table width="100%" border="0" cellspacing="0" cellpadding="0">
                            <tr>
                                <td width="25" bgcolor="#DCF2FF">
                                    &nbsp;</td>
                                <td width="892" valign="top" bgcolor="#DCF2FF">
                                    <div style="padding: 10px; text-align: left; height: 20px; margin-bottom: 10px;">
                                        <div style="margin-right: 10px; float: left;">
                                            <a href="addReport.aspx">
                                                <img src="images/tianjia_16.gif" width="113" height="23" border="0" /></a></div>
                                        <div style="margin-right: 10px; float: left;">
                                            <a href="#">
                                                <img src="images/tianjia_18.gif" width="113" height="23" border="0" /></a></div>
                                        <div style="margin-right: 10px; float: left;">
                                            <a href="#">
                                                <img src="images/tianjia_21.gif" width="159" height="23" border="0" /></a></div>
                                        <div style="margin-right: 10px; float: left;">
                                            <img src="images/tianjia_23_1.gif" width="159" height="23" border="0" /></div>
                                    </div>
                                    <div style="background: #ffffff; padding: 10px; width: 910px; margin-bottom: 15px;">
                                        <table width="97%" border="0" cellspacing="0" cellpadding="5">
                                            <tr>
                                                <td colspan="5" align="left" class="blue">
                                                    <img src="images/xiugai.gif" width="25" height="19" />
                                                    修改密码</td>
                                            </tr>
                                            <tr>
                                                <td colspan="5" align="left" style="background: #cccccc;" height="1">
                                                </td>
                                            </tr>
                                            <tr>
                                                <td align="right" style="width: 68px">
                                                </td>
                                                <td colspan="4">
                                                    &nbsp;</td>
                                            </tr>
                                            <tr>
                                                <td align="right" class="blue" style="width: 68px">
                                                    &nbsp;</td>
                                                <td colspan="4" align="left" class="title1">
                                                    修改密码</td>
                                            </tr>
                                            <tr>
                                                <td align="right" style="width: 68px; height: 32px;">
                                                    &nbsp;</td>
                                                <td align="right" class="blue1" style="width: 230px; height: 32px;">
                                                    <span class="red">* </span>请输入原密码：</td>
                                                <td style="height: 32px; width: 142px;">
                                                    <input id="txtoldPwd" name="oldPwd" type="password" class="input" />
                                                    <%--<asp:TextBox ID="txtoldPwd" runat="server" CssClass="input" TextMode="Password" onblur="txtoldPwdBlur(this)" />--%>
                                                </td>
                                                <td align="left">
                                                    <span id="span1" style="color: Red; border: 1px solid #ffcbcb; padding: 2px; height: 14px;
                                                        background: url(images/!.gif) no-repeat 3px; padding-left: 20px; padding-top: 5px;
                                                        display: none;"></span>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td align="right" style="width: 68px">
                                                    &nbsp;</td>
                                                <td align="right" class="blue1" style="width: 230px">
                                                    <span class="red">* </span>请输入新密码：</td>
                                                <td style="width: 142px">
                                                    <asp:TextBox ID="txtnewPwd" runat="server" CssClass="input" TextMode="Password" />
                                                </td>
                                                <td align="left">
                                                    <span id="span2" style="color: Red; border: 1px solid #ffcbcb; padding: 2px; height: 14px;
                                                        background: url(images/!.gif) no-repeat 3px; padding-left: 20px; padding-top: 5px;
                                                        display: none;"></span>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td align="right" style="width: 68px; height: 32px;">
                                                    &nbsp;</td>
                                                <td align="right" class="blue1" style="width: 230px; height: 32px;">
                                                    <span class="red">* </span>请重新输入新密码：</td>
                                                <td style="width: 142px; height: 32px;">
                                                    <asp:TextBox ID="txtnewPwdAgain" runat="server" CssClass="input" TextMode="Password" />
                                                </td>
                                                <td align="left">
                                                    <span id="span3" style="color: Red; border: 1px solid #ffcbcb; padding: 2px; height: 14px;
                                                        background: url(images/!.gif) no-repeat 3px; padding-left: 20px; padding-top: 5px;
                                                        width: 150px; display: none;"></span>
                                                </td>
                                            </tr>
                                            <tr>
                                                <td align="right" style="width: 68px">
                                                    &nbsp;</td>
                                                <td colspan="3" align="center" class="blue1">
                                                    <img id="imgok" src="images/login_27.gif" onclick="changepwd()" width="60" height="19"
                                                        border="0" title="确定修改密码" style="cursor: pointer;" />
                                                    <img id="imgcancel" src="images/login_28.gif" width="60" height="19" border="0" title="取消修改密码"
                                                        style="cursor: pointer;" />
                                                </td>
                                                <td width="29%">
                                                    &nbsp;</td>
                                            </tr>
                                        </table>

ie6测试，没有任何问题。
LZ脑残，鉴定完毕！

那是怎么回事呢？在firefox中都没有问题。

楼主 你用的是什么浏览器？ 我用ie6 也没问题 
是不是你ie浏览器出问题了 重新安装试试

你肯定调用blur 这方法js 死循环，
firefox 在处理js 调用有自己的一套，最多循环个几次也就不循环了。