Could be a multitude of problems from hardware issues to driver problems you can't just blame it on steam, if uninstalling steam doesn't work do the classic problem solver and re-install windows.
I'm guessing you've tried rebooting your computer, I'm just thinking along the lines of hibernating...
Man i remember watching the Portal trailer/demo when it was first released, i thought that looks 'cool'. Oh how wrong was I, I didnt grasp how awesome it was actually going to be/is!
Well put, i tent to push a lot as a sniper (given the right situation, i.e. the team is not desperately typing to fend off a charging horde) and not always successfully but its worth a go. There is nothing more annoying then a couple of camping snipers staying back out of the action. Its much...
Cant say ive noticed it myself, but ive not played a great deal recently. There was nothing about it in the updates. As for the whole sentry gun thing, maybe people have picked it up as a good tactic and naturally good counter tactics come along...
To quote Ziro Punctuation from the Escapist - "Yeah i know its not very funny to love a game, but F*** YOU! Portal is great and if you dont think so you must be stupid."
Wow helpful comment! I'm running Vista and ive had no problems playing the games, next port of call i would guess at is drivers, but evidently youve tried that already... go with the overheating business.