Garry's Mod starts up and then goes dead

I just got Garry’s mod for my birthday on the 30th, but when I try and start it up, it goes to the loading screen then it exits out and says ‘hl2.exe has stopped working and needs to close.’

I’m sure that I got most of everything for it to work, I got the Orange Box which lets me have HL2 and Episodes 1 and 2 with Portal, still in the process of downloading Team Fortress 2 since my internet’s a bit slow. Don’t have Counter-Strike: Source yet if it needs that to work.

If any of you can help me out here, I would be most grateful.

You need atleast a fully installed source game before you run GMod, and post specs always as it could be something else.