Assuming "a in painted screw" is a typo for "an unpainted screw"...
Yes, a mounting screw of a PSU that is plugged in and switched off is an effective ground.
Would this still cause static electricity to transfer since electrons transfer to what electronic you are working on since the power supply is not connected it it (like for instance a pc)?
Possibly, but you're probably going to touch the case of the other PC before you touch its internals, which should resolve any outstanding potential difference.