效果如下：
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
.code
{
background:url(code_bg.jpg);
font-family:Arial;
font-style:italic;
color:blue;
font-size:30px;
border:0;
padding:2px 3px;
letter-spacing:3px;
font-weight:bolder;
float:left;
cursor:pointer;
width:150px;
height:60px;
line-height:60px;
text-align:center;
vertical-align:middle;
}
a
{
text-decoration:none;
font-size:12px;
color:#288bc4;
}
a:hover
{
text-decoration:underline;
}
</style>
<script language="javascript" type="text/javascript">
var code;
function createCode() {
code = "";
var codeLength = 6; //验证码的长度
var checkCode = document.getElementById("checkCode");
var codeChars = new Array(0,1,2,3,4,5,6,7,8,9,
'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',
'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'); //所有候选组成验证码的字符，当然也可以用中文的
for (var i = 0; i < codeLength; i++)
{
var charNum = Math.floor(Math.random() * 52);
code += codeChars[charNum];
}
if (checkCode)
{
checkCode.className = "code";
checkCode.innerHTML = code;
}
}
function validateCode()
{
var inputCode = document.getElementById("inputCode").value;
if (inputCode.length <= 0)
{
alert("请输入验证码！");
}
else if (inputCode.toUpperCase() != code.toUpperCase())
{
alert("验证码输入有误！");
createCode();
}
else
{
alert("验证码正确！");
}
}
</script>
</head>
<body onload="createCode()">
<form id="form1" runat="server" onsubmit="validateCode()">
<div>
<table border="0" cellspacing="5" cellpadding="5" >
<tr>
<td></td><td> <div class="code" id="checkCode" onclick="createCode()" ></div></td>
<td><a href="#" onclick="createCode()">看不清换一张</a></td>
</tr>
<tr>
<td>验证码：</td><td><input style="float:left;" type="text" id="inputCode" /></td><td>请输入验证码</td>
</tr>
<tr><td></td><td><input id="Button1" onclick="validateCode();" type="button" value="确定" /></td><td></td>
</tr>
</table>
</div>
</form>
</body>
</html>