Ensure that PnkBstrA and PnkBstrB are both in your firewalls exception list.
You can find PBSVC here.If these are disabled or stopped or blocked by the user or anti-virus or Internet security software, updated PunkBuster servers will remove you from the game for Losing Key Packets. Users of some versions of McAfee Security Center are reporting that these PB components are getting automatically added to the "block" list. If this happens on your machine, you will need to remove them from the list in order to continue using PunkBuster. Players who have manually disabled or stopped PnkBstrA based on instructions from some other website may need to run our PBSVC program to uninstall and reinstall in order to continue playing on PB enabled servers.
To check that the services are running;
Click Start>Run>type 'services.msc' (without ''), click ok>look for PnkBstrA and/or PnkBstrB, make sure neither are disabled and both have started.
Also if your running the game in Vista, make sure you run it as an Administrator (right click the shortcut and select 'Run as administrator).