@mustang:
A.) Sorry we haven't figured out what's wrong yet, but Vista is not your problem. As I said, I play GMOD all the time.
B.) Try deleting your GMOD folder, the whole thing. It'll regenerate next time you start the game.
C.) In the main Steam folder, there is a file called ClientRegistry.blob, make sure Steam is closed and delete this file. It'll regenerate too. Doing this has solved nurmerous non-sensical steam problems.
@yodalman, yogadude, whatever your name is now:
You were IP banned for a reason, both your previous accounts were banned for a reason. Seriously now, I mean this 100%, grow up and get the fuck out of here. You haven't showed any majority since you used a proxy site and logged back on without an invitation. Isn't it clear someone, or multiple people don't want you here?