Garry's Mod isn't even working for me.

So recently, I made a steam account and bought Garry’s Mod and Team Fortress 2. After installing and updating all of them, I found out that Garry’s mod will not work. Which I find strange, since Team Fortress 2 work perfectly fine, just a little slow. When I start up Garry’s Mod, it begins with the usual picture of the guy with the bird and the physics gun. But as it seems to finish loading, it crashes, and a error box appears saying hl2.exe has encountered an error and needs to close. My friend who also plays garry’s mod and team fortress 2, had a crap computer, that couldn’t handle TF2, but played GMod just fine. My computer is far stronger than his old '00 xp shit, and its the exact opposite. I doubt that my computer doesn’t meet the requirements for Garry’s mod, but has this happened to anyone else or is there a way to fix it? I hope someone can help me out because Garry’s Mod is just the freaking best to play.

Posted this in the Fast Topics thread because i couldn’t find this, still don’t have an answer though.

Also, I’ve tried un-and reinstalling Gmod several times, but to no avail.

[editline]10th February 2011[/editline]

Also, I’ve tried that trick with the mount things, and that still isn’t working.

Try verifying your cache.