function abc() 
{ 
 var b 
  b=loginForm.User.value; 
 var a ;
  a=showModalDialog("包含CheckUser的页面.aspx?b="+b,"参数");
alert(a);

}

------解决方案--------------------

楼主可以变通一下,将CheckUser函数运算的值,赋于一个vb.net的全局变量,<%=这里直接返回全局变量%>
  <script type="text/javascript">
  
   window.onload=function()
   {  
      
      checkUser(<%="'" +sHello + "'"%>);
   }
  
   function checkUser(value)
   {
       if (value=="helloworld")
       {
           window.alert("isok");
       }
   }
  
   </script>

=============================================

namespace WebApp
{
   public partial class WebForm1 : System.Web.UI.Page
   {
       public string sHello = "helloworld";

       protected void Page_Load(object sender, EventArgs e)
       {

       }
   }
}