Just a quick note, for anyone having trouble playing games that have Punkbuster integrated with it. If you’re getting an “Invalid O/S Error” when you try to join a server, this is related to the game not having enough permissions to  open/alter certain files. There have been several fixes suggested, and some work for some people… some work for others. The one that seems to solve most people’s problem is running the game as an administrator. Simply being logged in as an administrator isn’t enough because your programs are still executed with limited privileges. You just have to confirm it when they’re needed(if you have UAC enabled).

To run as administrator you have a couple of options.. if you’re running a home version of Vista, you might not have some of them:

1. Right-click on the Shortcut or game executable and click Run as Administrator
2. Right-click on the game executable and Click Properties->Compatibility->Check the “Run as Administrator” box, Apply
3. Right-click on the Shortcut, Click the “Shortcut” tab->Advanced, Check the “Run as Administrator” box, Apply

Now, with that said, none of these would work properly for me. The only solution I could find that would work is downloading this update from PunkBuster, adding my game, and hitting Update. Once updated the game ran fine without giving it extra privileges.

Good luck!