It helped my alot.I figured that out my self.Now my game alsmost dont lag
1.Raight klick your desktop
2.Than go to Settings tab
3.than u see Color quality Its usaly seted to 32 bit
4.Just set that to 16 bit
5.Now by sure that ewrything is closed(xfire,firefox etc)
6.Now RUN COD4 and ingame options by sure that ewrything is on the lowest settings
7.Than go to play and enyoi
EXTRA:
If the 1 part didnt help much than try to download updates for your video card (I mean better drivers) And by sure that your comp is not full with some bullshiet
If nothing from that TUT didnt help than im sory U BETTER GET NEW COMP THAN