JavaScript: Which should I use, Microsoft.XMLHTTP or Msxml2.XMLHTTP?

You should definitely not use Microsoft.XmlHttp.

From the Microsoft XML Team blog: Using the right version of MSXML in Internet Explorer: ^(archive)

MSXML2 vs. Microsoft namespace – I’ve also seen a lot of code that instantiates the "Microsoft.XMLHTTP" ActiveX object rather than the MSXML2.XMLHTTP.3.0 or MSXML2.XMLHTTP.6.0 if you’re using 6.0. The “Microsoft” namespace is actually older and is only implemented in MSXML3 for legacy support. It’s unfortunate we used the “better” name on the older version, but stick to the “msxml2” namespace when instantiating objects.

Hai Cheeso,

Have a look at these
http://bytes.com/topic/javascript/answers/559991-msxml-xmlhttp-vs-microsoft-xmlhttp

http://forums.asp.net/p/1000060/1622845.aspx