<script type="text/javascript">
	function ajaxFunction()
	{
		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)
				{
					alert("Your browser does not support AJAX!");
					return false;
				}
			}
		}
	}
	</script>

Taken from here.