Please Help. Garry's Mod // Crash/Error for Windows XP.

[CrocodileTW]

Hello, I am having a huge problem. So all of my friends can apparently get on Garry's Mod but when ever I try to run it, I get this message:

hl2.exe has encountered a problem and needs to close. We are sorry for the inconvenience.

If you were in the middle of something, the information you were working on might be lost.

Please tell Microsoft about this problem..... And so on.

I have done many things to try and fix it.
I un-installed the whole game and deleted my whole garrysmod folder in steam under steamapps, and then re-installed the game.
I tried opening my task manager and ending the program and trying to re-open Garry's Mod, but it says I can only have one copy of the game open at a time.

I don't know what to do. If anyone could please help me it would be greatly appreciated.

-CrocodileThatWiggles