Sites that do this:
digitalpoint.com - Not XenPorta, but similar
I have found other threads where people were asking the same thing, but I haven't seen a direct answer yet.
Oh, and my host is HawkHost if that helps. I don't know exactly how to set up my subdomain. Do I set it's root to the root folder and then redirect it to domain.com/forums?
Any help is greatly appreciated, thanks!