There are two fixes for this. Please select the one right for you:

If you are under Admin and you still get this error

Try this file, it will check for adware that stops you from being able to play
on PB servers and fixes the problem even if you don't.

http://tools.zerosrealm.com/VX2Finder.exe

Click the “Find VX2.betterinternet info” button to make sure that you
don't HAVE this adware. If there are no files listed, go on to the last step.

If you find the virus, (it is usually 3 random named dll files)

Select all the files found.
Press ‘Delete These Files’.

The program will delete all files but one that will be deleted on reboot.
Allow program to reboot.

Once Restarted:
Press ‘Guardian.reg’.
Press ‘User Agent’.
Press ‘Restore Policy’.

Clicking on “find vx2.BetterInternet info” again should show all
fields blank.
If you have no adware files, just click on the “Restore Policy” Button

The other fix is:

Type these commands in the console:

*From the Main Menu, press the Tilde “ ~ ” key to open the console.
*At the prompt type:

/pb_cl_enable (press the Enter key)
/pb_sv_enable (press the Enter key)
/pb_sv_update (press the Enter key)

*PunkBuster will now begin to update. When it is complete you will see yellow
text providing the new version numbers.
# Close the console.