Problem with Start Up

Disclaimer: Yes, I know there’s already a thread on this but may situation is very different.

I’ve played Gmod multiplayer going on 1 1/2 years. It’s probably better than a lot of games I have for consoles, but now there’s a problem. Barely two weeks ago, my computer was hit by a virus, it was one of those advanced fake anti-virus ones (like something that would put Geek Squad up for its money.) It fucked my computer up right quick, luckily I got some help and got it out of my system, but a lot of other files I had to delete. So now I try to start it up my screen just goes black for two seconds and it goes back to the desktop. I need some help here. Any help will do. Thanks!

UPDATE: I tried to redownload Steam after deleting random files from my Steam folder in program files. After trying to reinstall Steam I failed but the Garry’s Mod icon was still there, so I double-clicked on it and I got Steam re-downloaded for some weird reason. Then I installed Garry’s Mod again (with no local content present, it started at 0%.) After downloading and trying to play the game, it still didn’t start up. Then I tried to download Counter Strike: Source and tried to play that (since it seemed like I start a CS:S match first and then I can play Gmod) but in CS:S it just played the Valve animation and closed itself. I need some help with this otherwise I’m going to go mad.

[editline]November, 2nd 2011[/editline]

How did no one fucking respond yet?