Hi Greycell,
It seems like you are having some problems with the RAM on the GPU - do you have the latest patches for COD4 - it is a very GPU intensive game and you really need a more powerfull card to run it on max settings -
try and take all the higher setting off and run it at your standard resolution and see if that helps - if the game works then the chances are the GPU can't handle the shader effects etc, you will find this is more common on 64bit OS's as they render more of the background.