half life / gmod both not loading

[lost1010]

ok, heres the problem, i just got Gmod and HL2 and when i start either of them up they screw-up. half life 2 gets into the game past the first picture then sticks itself at the loading screen (so i havn't even reached the main menu). And, Gmod get kinda into the game but when you try load up a level it get to something like "loading resources" then literally restarts my computer.... i have been searching through everything i can find on google and in forums for about 7 1/2 hours so i decided just to ask. i have all the lates video drivers for my GeForce 8500 gt installed and have tryed everything on the internet that was related sort of to my problem but still i havn't budged an inch further after 7 1/2 hours of no sleep.

any1 got any tips?

quick update: i tried HL2 after re-installing DirectX and got this error

failed to lock vertex buffer in CMeshDX8::LockVertexBuffer

and im searching up about that error