Yank a key off of a Win 7 PC that someone is tossing and install Win 7. MS isn't all that strict. Personally, if it's just being used for QB and a few other things, I'd use XP till it drops or is no longer supported by QB, have good virus protection, and not be a cretin while browsing.

Who gives a crap if it's no longer updated as long as it does what you need it to? I've seen proprietary systems that still use NT or Win2k.