PnkBstrA will be displayed in the services.msc menu... if you've installed Punkbuster. PnkBstrB will be different dependent upon which version you have: 32-bit or 64-bit.
64-bit PnkBstrA and B reside in the Windows/SysWOW64 folder; you should set them both to run as admin to avoid problems.
32-bit will be in the Windows/System32 folder; again, let them both run as admin.
You may also find PnkBstrB in a game's pb folder... dependent on the game. For example, I have PnkBstrB in Battlefield, but not CoD 5.
While I have PnkBstrA&B set to run as admin in the SysWOW64 folder, which should be sufficient, I have also enabled them to run as admin in whatever game directories they show up in.
Hope this helps?