This ones from Tally over @ COD utilitys
If you are referring to the infamous "iwmp.exe has stopped working" error, then there are several causes for this - (as mentioned) sound problems; User Account Control problems; bad install problems (files corrupt).
1. For User Account Control problems, right click on the COD4 iwmp.exe and click on the "compatability" tab. Make sure you have "run this program as administrator" checked, and also make sure you check "run this program in compatability mode for ... XP SP3".
2. For sound problems, make sure you have all your sound settings correct. Disabling a device which was enabled when you installed COD4 can make the game fall over with an error. Make sure all devices are setup correctly. Also, make sure your sound driver is not corrupt. In fact, any kind of problem with sound will cause COD4 to fall over because it uses Miles sound technology and is wholely dependent on Windows sound setup being correct.
3. For a bad install (and files do get corrupt in game installs from time to time), just reinstall the game. However, this should NOT be the first thing you try. If its another problem, reinstalling will not likely fix the problem because it will still be there after you've reinstalled.