During the past 7-8 months Activision have released a number of patches for COD4 via the Steam distribution. These are to solve vulnerabilities in the code, the outcome for many users was the loss of favourite server access.
I strongly recommend that if you are playing COD4 you do so with the COD4X executable, this adds lots of features that make it more secure and more fun to play the game. The file is available at the bottom of this post, to run this on a Steam install you will need to manually roll back the COD4 MP exe to the pre patch version(also attached to this thread). After doing this turn off auto updates for COD4.
Our servers run the COD4X exe and have done for a couple of years now, it is great.