In Source I get an avg of about 20-30 fps and sometimes in some areas or when theres little people 40-50. In the video stress test I get about a 50-60fps avg.
My specs are:
Intel P4 2.4 GHz
ASUS P4SD
2x 256mb DDR Dual Channel
XFX GeForce 7600GT

Can someone tell me why my fps is so low? is it my specs?

yeah it could be your specs actually. you should try running the game with lower settings. try this link...

http://forums.steampowered.com/forums/showthread.php?t=523089

you should also use your own custom config if you know how to do it. if not i recommend this config...

http://forums.steampowered.com/forums/showthread.php?s=&threadid=453432