Cant play CoD4 Multiplayer! ( Singleplayer works fine )

help!!
i cant play CoD4 on multiplayer for some reason. i just got a Radion 3650, and it plays singleplayer at max settings.
but when i start multiplayer, my screen goes black and then i get a message saying

"iw3mp.exe has stopped working"
Check for a solution Online
Close program

and thats it!
please help.
heres my specs:

eMachines T5088
1.5 gigs of ram
Intel Pentium 4 3.2 GHZ
ati radion hd 3650

heres my computer:
http://www.emachines.com/products/ [...] prod=T5088
but i upgraded to 1.5 gigs of ram, and a ATI Radion HD 3650

Ok, you have Vista right? I just installed Vista on my secondary computer and ran into the exact same problem. It's actually an easy fix:

-Control Panel
-Sound
-Recording tab
-Right click in the box and select "show disabled devices"
-"Stereo Mix" should then appear
-Right click it
-Enable
-Apply
-OK

Don't know why that works, but it does. By the way, my control panel is set to classic view so it may be a little different on yours. But you get the idea.

I'm having the same problem too. Single player works fine, everything on high. Then i try to go to multiplayer and i get the same error. Vista x64 sp1. q6600 4800x2.

Ill try the above solution tonight also.

ok

crud.....

ok, reinstalled windows, installed cod4

followed the above tip and it does work perfectly now