If a site is written in ASP, does it have to be on a Windows server or can we use Apache? Which one is best and why?

Sorry if this is posted in the srong place, I wasn't sure where it should go :s

Nope, it's the right place.

Windows server, since ASP is a Microsoft programming language. ASP is native to the Windows platform and therefore performs better on it (regardless of what people tell you about ChiliASP or whatever variant goes on an Apache/Linux/Unix/whatever server.)