var xmlHttp;

function createXMLHttpRequest()
{   
      
   if (window.ActiveXObject)
   {
       
     xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
   }
   else if (window.XMLHttpRequest)
   {
      
     xmlHttp=new XMLHttpRequest();
     
   }
}

function valUserName(ref)
{
 
  if(ref.id == "txt_UserID")
  { 
    
       // var userName=document.getElementById("txt_UserID").value;
       var url="Forum/ValRegister.aspx?userName="+ref.value;
   }
   if(ref.id == "txt_Email")
   {  
      
       var url="Forum/ValRegister.aspx?userEmail="+ref.value;
   }
   createXMLHttpRequest();
   xmlHttp.open("GET",url,true);
   xmlHttp.onreadystatechange = handleStateChange; 
   xmlHttp.send(null);
}

function handleStateChange()
{
   if ( xmlHttp.readyState == 4 )
   {
     if ( xmlHttp.status == 200 )
     {
          
        parseResults();
        
     }
  }
}

function parseResults()
{
  
  var results = xmlHttp.responseText;

  if ( results == "isUserName" )
  {
      alert("userName exists");
  }
  if ( results == "isUserEmail" )
  {
      alert("userEmail exists");
  }
}