I need urgent help!

When I start Garry’s mod on steam, it launches normally, then after 4-20 seconds, it crashes and gives this engine error :


I Dont know what to do, I love this mod, I love this community(well most of it,) and i want to keep playing. So, please Help.



Oh, i forgot my Computer Specs :
AMD Athlon 64 3800+
1GB ram
Geforce 8600GT+
Windows XP