http://www.microsoft.com/downloads/release.asp?ReleaseID=37176

MSXML 4.0 SP1 offers a number of new features and improvements over the MSXML 3.0, including support for the XML Schema language and substantially faster parser and XSLT engine. MSXML 4.0 SP1 is a complete replacement for MSXML 4.0 RTM and provides a number of bug fixes. It can not replace MSXML 3.0, because some obsolete and non-conformant features are no longer supported. MSXML 4.0 installs side by side with MSXML 3.0 and earlier, without any interference.