If you run any PB protected games under VISTA in a backward compatibility mode, your PB services cannot run with admin rights anymore. Therefore you will either experience a kick from PB protected servers or freezes, which are in fact the same thing. The A service might be started well but is not entitled to call the B service once you connect.
There are some games/compatibility mode combination (such as running as Win2000) which will allow PB to communicate again, but you may experience other issues with the particular game itself, such as no sound, bad graphics etc. ...
Like Roadwarrior said before, running games under Vista in comp. modes will cause you troubles and does not represent an option at all. There should be no reason to run Cod4 under WinXP emulation, just allow it to run as administrator and it will do fine under Vista.