I had this problem when I upgraded from my GeForce FX 5700U to my current 6800GT. The only thing I did, and I am pretty sure this will work for you, is to go into the
"SteamApps\Half-Life 2\hl2\config" (I don't know if thats exactly right, but it should be close) directory within the place you have HL2 installed, and delete the config.cfg file. This will force the game to redetect all of your hardware, and this should work. If not, you may just have to reinstall the game. You can do this with all of the other games (CS:Source, HL:Source) to get them to work as well.
If you DO have to end up reinstalling the game, get your savegames from:
"SteamApps\Half-Life 2\hl2\SAVE" directory, and copy everything to a safe location on your hard drive. Then, after reinstalling, you can just copy everything back over into this same directory.
Sure hope this helps.