Well, you don't exactly install IIS "on" Apache, for say, but you can install them both on the same machine and run them side-by-side (granted you change some ports around, as they cannot run side-by-side when both are using the standard port 80).
Secondly, thanks to a 0.25 second Google query, using the tag "how to install IIS on vista", I found this step -by-step guide on the matter:
How to Install IIS on Windows Vista :: the How-To Geek
Google is your friend
-Chase-