try running in safe mode...then quit the game and run again in normal mode...if this doesn't work it could well be that your system is too old unless uber tally or hgen knows what this is about.
Sounds like your system mate, I had the same thing with original CoD, upgraded my PC to 768 RAM, GeForce FX5600XT card and the games run sweet as nut now, even CoD2 on MP runs fast, I overclock my graphs card sometimes aswell to run the game on 1280 x 1024 rez, still works well. If you are going to upgrade now, I would personally (depending on how much money you have ) go for either the Nvidia 7800GTX or ATI 1900XT they are plenty bucks, but kick ass cards. or even wait till Nvidia bring out their next card in a couple of months. - Good luck