After installing all device drivers needed and rebooting in between everything looked and worked all right.
But at the end of the day when I tried to shut down my PC the problem became clear:

The complete PC and all devices turned off as they should except for the Power Supply Unit.
Somehow the PSU fan keeped turning in an 'on' state but I could hear all devices like the hard disks turn off.

After some google time I found the solution to the problem.
The Asus P5B serie mainboard has a bugged firewire controller.

The solution is quite easy, just enable the option 'Allow the computer to turn off this device to save power' in the firewire/1394 device in the Device Manager.
This enables the PC to turn it off also at shutdown and solves the problem of the device preventing a complete shutdown.

Tagged: hardware, mainboard, asus