function createXHR() 
{
    var xhr = null;
    try
    {
        //W3C
        xhr = new XMLHttpRequest();
    }
    catch(e)
    {
        try 
        {
            //IE6+.
            xhr = new ActiveXObject("Msxml2.XMLHTTP");
        } 
        catch (exIE6)
        {
            //IE5.
            try
            {
                xhr = new ActiveXObject("Microsoft.XMLHTTP");
            }
            catch (exIE5){}
        }
    }
    return xhr;
}