I Cant Shut Down !

Hi there.

Yesterday i opened the case of my pc to do some dust removal.
I used a clean brush for this purpose, whipping away dust from the
motherboard.
After i reinstalled what i had taken out, following Problem occured:
I startet Windows, did some work and when i wanted to power off the pc
(with Windows-Shut down ), the pc would power down, but imediately
after the
fans stop rotating the system would power on again !.
So i went and disconnected every component from the motherboard but the
CPU and the PSU ( eben the RAM), but the problem persisted.
I used another PSU from a friend, connected it to the motherboard: still
the same Problem.

Has anybody a hint, why the system still wont power down ?

Asus A7v333
AthlonXP 1900+
350W PSU

Jonas