The minimum suggested requirements for WebMatrix hosting are Windows Server 2008 R2, IIS 7, and both ASP.NET 4.0 and 3.5 available (
How to config the host for webmatrix?[
^]), but in my experience Windows Server 2008 is enough.
If you want to try to install WebMatrix on IIS 6 this thread could help:
WebMatrix w/ IIS 6?[
^]