Shadow colour problems

[Jambi]

Hey well basically my problem is that in half life 2 when there is a very dark shadow in say the corner of a room in nova prospekt or something there is a blue, red or green colour to it and it looks really really bad. I was wondering how to fix this, i tried reinstalling the game didnt work though, Half life 2 episode one doesnt have this problem nor does source.

My computer specs;
AMD Athlon 4200+ 64 X2
2 GIG RAM
GeForce 7600 GS 512 ram

Not a bad computer really, and my monitor is
ViewSonic VP720b
4ms
I run the monitor res 1280x1024 Refresh rate 60hz

any help would be greatly appreciated thanks

~matt