COD4 help please

so I just got COD and it wont start when I try to play online, the screen goes black like getting ready to switch to the game and then it tells me ''iw3mp.exe'' has stoped working and then it says you need to close the program and try to start again, I have been doing that like a 100 times already and it does not work. The offline works great at 1680 by 1050 but the online does not work it gives that message all the time is anyone else having this problem, and how can I fix it