Unfortunately Web Matrix installs II7, compatibility items for PHP under IIS (IIS' native programming language is ASP -totally unrelated to WordPress, which is written in PHP), Microsoft SQL Server reporting tools (even though WordPress uses MySQL and not Microsoft SQL), Microsoft SQL server (which is no small program) along with a whole plethora of other compenents. Uninstalling all of that is a challenge too. Performance is also abysmal. There are other issues too, as Microsoft's platform installer servers may not be online, or working correctly.
Microsoft is making progress on this front however, and Windows8 promises to include much WordPress friendly integration. The platform installers might be fixed someday soon too.
Did you want to wait for that to happen or perhaps install a more compliant stack that WordPress was originally developed on and for, such as Apache, MySQL, and the original PHP interpreter? Consider ServerPress.com or InstantWP instead.