function check_user_availability()
{
var xmlHttp;
  try
    {    // Firefox, Opera 8.0+, Safari    
	xmlHttp=new XMLHttpRequest(); 
	}
	catch (e)
		{    // Internet Explorer    
			try
			{      xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");      }
				 catch (e)
						{      
						  try
							{        xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");        }
								catch (e)
								{        return false;        } 
							
						}    
		
		}

xmlHttp.open("POST","/cgi-bin/AddBook/check_user_availability.pl",true);
xmlHttp.send("User=" + document.user_form.User.value); 

xmlHttp.onreadystatechange=function()
      {
	
      if(xmlHttp.readyState==4)
        {
		  document.getElementById("user_availability").innerHTML=xmlHttp.responseText;
		
        }
      }
    

}