http://forums.steampowered.com/forums/forumdisplay.php?f=80
I suggest going there. The forums are a great place for finding help, and it's a lot quicker than asking Valve. I once had a problem and it took Valve over two weeks to respond (I like them, they're a good company.. just slow).
The only thing I can think of is resetting Steam, making sure there aren't any updates you might have missed or need downloading. There's a specific file in the TF2 folder that you can delete which will force the game to update it self when you start it up, check on the forums for which one it is...