I bought CoD: Modern Warfare on Steam not so long ago only to find that I got a Punkbuster is not initialized error whenever trying to start up multiplayer. After a bit of digging, I found a fix.

Go into your Call of Duty 4 directory. If youre running Windows XP, it is likely at the following path:

C:\Program Files\Steam\steamapps\common\call of duty 4\PB

Run the program pbsvc.exe from this path and let it do its configuration mojo. It takes a few minutes.

Download a new version (3.4) of pbsetup.exe from the [ Register or Signin to view external links. ]

Run the new Punkbuster setup, click Add Game, and select Call of Duty 4.

Now click Check for Updates. It should download new Punkbuster files for CoD4.

Now, just run COD4 and join a server.

Congrats, your Call of Duty 4 multiplayer should now be fixed.

Any more problems/errors? Post below and I'm sure we can fix it - or drop me a PM!