However, neither CoD4 nor MW2 for PC have default control mapping for the Xbox controller. You might be able to use some clumsy software to make the controller trigger keypresses, but it doesn't work that well. But TBH, many gamer use the PC to play game to be able to use the mouse and kb. Perhaps you might like it as well?
I'v got software in my system preferences called x-box 360 controllers.. can't remember where i got it but it works fine for most games with my controller. try googling for it