Games running much slower in Win 7 than they are in OSX.

I've been using bootcamp for two years, absolutely zero problems. Then a few days ago I decide to do a fresh install of OSX as it was getting a bit bogged down by random junk and such, and as a consequence, my Bootcamp partition was deleted. I've reinstalled bootcamp and now I notice that games are running PAINFULLY slowly. On OSX I can run SCII on all low settings, but on Win 7 Ultimate 64-bit it refuses to run on minimal settings in the lowest possible resolution. It's the same with DOTA 2, League of Legends. I used to be able to play Tribes: Ascend and Blacklight: Retribution and all of them are now unplayable. My brother's laptop with the HD4000 is running them on medium settings without so much as a hint of complaint, whereas my computer can't even run them on the lowest settings smoothly. I have the MBP with the HD3000, surely the performance gap isn't that much? I know it's not that I can't run these games, as I was playing DOTA 2 hours before I reinstalled Bootcamp and it was running like silk, and after my reinstall it refuses to run properly. OSX is running fine and the Desktop environment in Win 7 also seems to be doing ok. I've checked all my display drivers and they are all up to date. Any help would be greatly appreciated.