[nuts, posted without comments, sorry...]
Identical problem here, so this is not an isolated incident, it is not limited to Vista, and (at least in my case) it was not caused by moving groups within PWM. Experienced when running PWM FF extension (fault started with v1.72b1, then b2 now 1.72) on XP SP2 and SP3 machines. Running FF3.X0b prior to problem (not FF 2.x.) PWM seemingly broke with 1.71b on FF 3.0b4.
Exporting PWM settings, importing on a different workstation, and then clearing and reinstalling PWM and manually recreating entries do not resolve issue. Problem persists even with newest PWM 1.72 (non beta.) PWM extension on FF 3.01 is broken on my XP workstations, and reinstalling FF/XP is not an option for now.