You haven't re-installed in 3 years? That's a big mistake for gaming, if you ask me. Unless you're incredibly anal about keeping it clean.
I re-install at least once every 6 months - because I hate the feeling that everything is not optimised. So, to me, it's a trivial process. The longer you wait, the worse it'll get
I've just been too lazy and cheap to do it until now.
What's the advantage (if any) of paying the extra money for the full version vs the OEM version? Besides the fact that you get both 32bit and 64bit discs with the full version.