OMFG Cobra I told you how to fix it once!!! Either A. put your CD in. Don't know what that does but it works for me if I'm having that problem.
Second thing is to B. close down your internet and restart it. If you have a router or something try reseting it. Maybe not reset, but just try restarting your internet. If neither one of those work, then you're on your own.
That is a thing that I have been trying to get a solid fix for forever!!
I almost think its related to firewall settings or something. Not quite sure but it has something
to do with your connection to the CoD4 Master Server. I've played around with firewall settings and that fixes it sometimes, but other times it doesn't.