Cod Mw Multiplayer stopped working error.

Hello,
Guys I installed my Cod and the singleplayer works fine but whenever i click on the multiplayer icon an error pops up saying Cod has stopped working.I googled this error and found a lot of guys saying to enable the stereo mix but there is no such thing as stereo mix in my sound options and also I dont have a microphone, just a pair of handsfree. Please help
P.S I have 64 bit operating system and no antivirus installed so does that matters too?