Garry's Mod crashing, never happened before?

I recently got my computer back from my parents, after about a year of having it taken away for school reasons… I used to always play Garry’s Mod and it worked great. I just set my computer up yesterday, everything the same as before. I downloaded Garry’s Mod last night. I’ve tried loading it up like 5 times now, ad every time i load it up it crashes before getting to the main menu. It runs at first, and it shows that picture of the guy with the cone on his head, then after about half a minute, the monitor goes off, then the picture for intel or something (no fuckin clue but it always worked before, i have a great graphics card and enough RAM and everything, garry’s mod ran smoothly in the past) pops up and then my computer reboots. When it loads back up and gets to the desktop, a message saying a serious error occured. It doesn’t say what kind of error. What’s wrong??

Oh and Counter-Strike Source works great I’ve been playing it

Edited:

How do I find out what drivers i have and what updates to get?

Edited:

Sorry i feel stupid. It’s been a long time ha

Well check your drivers and download the latest ones,
If not then Try defragging or Verifying your Game cache.

Try verifying integrity of the game files in Steam.

[editline]26th February 2012[/editline]

Ninja’d

How do I find out my drivers? And I’ll try the other things on steam

Start > Run > DxDiag and click on the Display tab.