Game startup issue


FS Member
Ok.. so i installed steam
I ran "Install and Launch"
I Installed HL2
I Installed GarrysMod
I Installed DOD:S

Now... When i try to start Garrys Mod, It loads, "Garrys Mod" appears on the task bar, but then it closes... It doesn't even enter the game.

When I try to start Day of Defeat : Source, <DITTO^> , it shows the intro, but then it closes.... ... I have no idea what may be causing this error, can it be AVG? the Antivirus? It's really frustrating.... Please help, if you can.