Garrys Mod Not Working At All. Please Help.

I just got back from Basic Combat Training for the US Army and i had some cash so i got a 2 G computer and when i hooked it all up i tryed to play GMOD and it just shows the icon at the bottom of the screen then shuts the program down. My computer has Win 7

Steam ID

Creamator101 (Alpha Worrier)

Specs? - the same computer