Windows can run php but there are slight differences in things like path names which can cause some applications not to work properly and also php is often not standard on windows hosting so you often have to go for a asp based forum. You have to pay for the software and also a share of the operating system costs and database license.Please explain why the (bad) choice of a Windows platform affects the choice of forum software.
With Linux you can't run asp but php is standard and a lot more stuff is free such as phpbb, the operating system and database server.