I don't know where to lead you on this. It comes down to whether or not you remember when it was your problems began. You seem to have done what you needed to do as far as drivers go, so thats probably not the problem... although it still could be.
I would say that something with the update of Sp2 changed your machine. If you did go from Sp1 to Sp2, then it was a major upgrade. You now have windows firewall running at all times which could be eating resources while you're playing your game. It's hard to say.
If you're running system restore, I would try to go back to a date when you suspect your machine was working fine and start over from there. Other than that, I'm kinda out of ideas. Sorry.