Startup problem

Whenever I try to startup GMOD, I get this error:
Garry’s Mod: hl2.exe - Application Error
The instruction at “0x00010103” referenced memory at “0x09bd12e4”. The memory could not be “written”.

Help would be very appreciated, considering I owned the game a while back and just re-downloaded it. I bought it with Team Fortress 2 and installed it.

Run memtest. Last time I got that error, something was wrong with my ram.