Did you already try backing up your configs/hud/demos/other things, and then delete the "team fortress 2" folder in /steamapps? I don't think you'll have to re download the whole game, it should just extract a fresh copy from the GCFs.
EDIT: As others will probably say, TF2/Source is most likely bottlenecked by your CPU if a fresh copy doesn't perform like it should. You could try overclocking your CPU if you're comfortable doing so, it should be rather easy with an unlocked Phenom.