Go to this youtube video
COD4 Get High FPS on any Computer. - YouTube
it increased my fps by 2-5

also try this:
type in the command
/exec dko5.cfg
NOTE:This will change your controls so before entering this, Remember the default controls
For you lazy People out there here are the controls for moving and other stuff

Moving Forward=W
Moving Left=A
Moving Right=D
Moving Backward=S
Jumping Over Things=Space
Crouching=c
Prone(go on the floor)=Ctrl
Aim down sight=Right Click
also your FPS Counter and Lagometer should appear so to make it disappear just type in
/seta cg_debugInfoCornerOffset "0 0"
Also try lowering your Resoultion to the Resoultion Your Graphics card has(yes i know its suppose to be to lower into the lowest it can go)
post if it helped!!

also i know some of you might say to also put gamebooster but acutally gamebooster does give more fps but it makes you lag more and your ping go high