Yea this is a problem. Your computer is great. Are you having this problem with any other games or just COD5. If the answer is no than its probably the other players that are affecting you. High pinged players usually make you lag. Look for online servers as you do, and order the list in order of PING value. Pick the lowest server and see how it goes.
If other games are laggy its probably your graphics card. You probably need to reinstall them and COD5. You may need a lap top fan to cool off your computer also because it might be overheating.
Also try ending some processes that arent needed while your playing COD5.
Try defraging your hardrive also.