Hey mate this is from the infinity ward website, i hope it helps
I cannot run iw3mp.exe / iw3sp.exe it errors with 'iw3xx.exe has encountered a problem and needs to close'
There are a number of possible reasons for this happening:
Some onboard soundcards - Start ->Control panel -> Sounds and audio devices-> options -> properties. Pick Recording and then turn stereo mix ON.
Realtek onboard soundcards - Make sure you are using the newest drivers available. Some have reported that having a mic plugged in can fix this problem.
Realtek onboard soundcards - Sometimes disabling / enabling these can make it work (even if you dont use them for sound)
Realtek onboard soundcards and vista - Sometimes uninstalling the realtek driver and letting vista find its own works.
AthlonXP and Sempron (not 64bit) - Go into the cod4/miles folder and rename 'mssmp3.asi' to 'mssmp3.bak'. The miles sound system included is known to have issues with 32bit processors. NOTE: This will disable ambient music in the game. An alternative mssmp3.asi can be found Here! so you can have music.
Dual X2900xt in crossfire mode - Downloading the WDM driver from the Ati/AMD Site is known to fix this.
As always if you have problems, Try to update all drivers (motherboard, soundcard, graphics card) to the newest available. Most can be found at your motherboard manufacturers website ( www.asus.com www.msi.com.tw www.via.com.tw www.gigabyte.com.tw etc.)