Good news: It seems like taskmanager (Ctrl+Alt+Del) actually is working, it`s just hidden behing the OPF screen
So hitting Ctrl+Alt+Del, waiting a few sec, then hitting Alt+E, waiting a few more sec and hitting Alt+E again does terminate OPF on my Win XP system.
It`s not a perfect soulution, but at least I dont have to reboot all the time.