注册页面：register.htm
<script language="JavaScript">
function GetResult(str)
{
    var exitdos = new ActiveXObject("Microsoft.XMLHTTP");
    exitdos.open("POST","check.asp?userid="+escape(str),false);
    exitdos.send();
    document.all.username.value=unescape(exitdos.responseText)
}
</script>
userid:<input type="text" name="userid" onblur="GetResult(document.all.userid.value)"><br>
username:<input type="text" name="username">

验证页面：check.asp
<%@Language="JavaScript"
function OpenDB(sdbname)
{
    var connstr = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source="+Server.MapPath(sdbname);
    var conn = Server.CreateObject("ADODB.Connection");
    conn.Open(connstr);
    return conn;
}
var sResult = "";
var oConn = OpenDB("data.mdb");
var userid = unescape(Request("userid"));
var sql = "select username from users where userid='"+userid+"'";
var rs = oConn.Execute(sql);
if(!rs.EOF)
{
    sResult = "该用户名已经存在！"
}
else
{
    sResult = "该用户名可以注册！"
}
Response.Write escape(sResult)%>

mdb数据库名：data.mdb
表：users
表结构：id(自动编号)、userid(文本)、username(文本)

到此，源代码和数据库就完成了，因为用的是onblur事件，当光标的焦点离开时，input值会马上传到数据库验证。