var xmlHttp = false;
try
{
	xmlHttp = new XMLHttpRequest();
} 
catch (trymicrosoft)
{
	try
	{
		xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
	} 
	catch (othermicrosoft) 
	{
		try 
		{
			xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
		} 
		catch (failed) 
		{
			xmlHttp = false;
		}
	}
}

if (!xmlHttp)
	alert("Error initializing XMLHttpRequest!");