Gmod, and other source games, won't run

I recently bought Gmod, and installed it, all through Steam.

I had previously purchased CS:S and ran it, and played it for a bit. So obviously, source games will run on my computer.

However, after I installed Gmod, no source games work. When I try to run Gmod and DoD:S, they begin to open for a split second, and then close out. No error messages. Nothing.

CS:S won’t even get that far. It “prepares game” and that’s it.

Any ideas?

EDIT: The games that I have installed in steam are: Counter Strike 1.6, Counter Strike Source, Day of Defeat, Day of Defeat Source, HL2-Deathmatch, HL2-The Lost Coast, Peggle Deluxe Demo, Peggle Extreme.

My computer meets all minimum requirements for the game, and all drivers are updated.

bumpity.

Don’t bump a thread less than a day old…