ummm a 7300 GT should be able to run COD4 for sure when it comes to compatibility. i imagine the fps is pretty low though. but if you were running it before and then it stops working all of a sudden... you have a different problem. have you messed around with the video/graphic settings in the game itself? i have had that problem before on a few games, and i ended up having to change the settings by opening up a config file in note pad and changing the settings from there to get it to work.
do other games still work on your pc just fine? if they do then that just proves to us that it is not actually directx causing your problem. from the amount of info i have for your current problem the only thing i could suggest at the moment is to try a reinstall and see if that works. if you want to keep all of your progress, just remember to back up all of your savegame files before you try the reinstall.
