There are a couple of ways to achieve this, but one of the most popular is the URL Rewrite modifcation available here:
http://cheeso.members.winisp.net/dl/...piRewriter.zip
You may need to consult the online documentation, though I hear the installation is reasonably straight forward.
I'm more accustomed to using Apache so cannot go too far in depth without having a look at a Windows server myself.
Any problems, let us know.