I had a problem with the heartbeats stopped problem from pnkbstrb.exe and very slow performance from Battlefield 2 Special Forces, i fixed all these problems by doing the following.
Turning off ALL firewall services
Turning off Windows Defender
Uninstalling and reinstalling Punkbuster
and this final step is the one i found odd as it was last thing i tried changing and suddenly all the problems with Punkbuster went away (slow game performance, and punkbuster error messages handshaking, heartsbeats stopped)
I went into the game's video card settings and set all settings to HIGH, i also set the video cards setting to highest settings within the video card's management software.
After i changed these video settings and dont ask me why ALL my problems with punkbuster went away and now punkbuster runs as if i never installed it!
:)