Frequently Asked Questions about PunkBuster for Call of Duty 2
Frequently Asked Questions about PunkBuster Services
Most likely your Client side PB software is not working properly, try to download pbsetup from here, run it, if the problem is still present, do this:
copy the pb folder you have and rename it to pb_old or similar, then make a new folder named pb and move the pbsetup tool in there and run it.
That should and hopefully solve your problem.