"Bad serial number given in setup" error

There are several possible fixes for this issue.

First, run Arma 2 once. Then try to run Arma PMC (without beta or DayZ).  If that won't help you can try to verify game cache of the Arma 2 and Arma OA.

1.) If that will not help, follow this guide:

1. RUN STEAM AS ADMINISTRATOR
2. Exit Steam.
3. Right-click on Steam icon.
4. Left-click on "Run As Administrator".
5. If the problem persists, reboot your computer and try the procedure again.

2.) DELETE ARMA 2 PMC REGISTRY ENTRIES

1. Exit Steam, open Start menu and type this directly into the menu (in Windows XP, you will have to click on "Run" first) regedit
2. Press Enter.
3. In regedit window, click on HKEY_LOCAL_MACHINE.
4. Press CTRL+F to bring up search.
5. Look for "ArmA" (without the quotes).
6. Delete all the "ArmA" entries you find from the registry.
7. Run Steam as Administrator.
8. Right-click on the game in Steam Library.
9. Click on "Properties".
10. Select "Local files" tab and click on "Verify integrity of game cache files" button.
11. Please wait, the process can take several minutes.

3) REINSTALL STEAM

1. Go to Steam's installation folder.
2. Delete everything there except "steam.exe" and "steamapps" folder.
3. Run "steam.exe" - Steam will reinstall itself automatically.