This worked for Dreper:
1. Download
>>PBsvc.exe<<. Place it in the Activision/Call of Duty 4/pb folder. It is imperative you put it here!
2. Run pbsvc.exe and choose Uninstall.
3. RESTART after uninstall. You will continue to have trouble if you don't!
4. Run pbsvc.exe and choose Reinstall.
5. When that is completed, download
>>PBsetup.exe<<6. Save it to some place handy, as you need to manually update Punkbuster sometimes.
7. Run PBsetup.
8. Click on Add a game and select Call of Duty 4 in the game box
9. In the box beneath select the path where the game is installed
10. Call of Duty 4 should be listed now
11. Press the "Check for Updates" button
Now run the game, and it should work!