First of all: Thanks for all the responses!

I think thats its an BIOS related problem aswell, my father with a different hardware has the same problem but only sometimes.

I'll check out the boot options and make HDD first before CD/DVD.
But: There are no USB drives connected, no LAN cable connect (drivers are installed for that, but no cable connected), WLAN card (PCI) is not installed yet.

@Christian_A:
The computer sleeps before the OS starts loading.

1. BIOS/Mainboard boots
2. idle (sometimes with visible boot screen of Windows)
3. OS boots


edit #1:
Just changed some boot configuration to display the messages of the motherboard and deactivated the fast booting for testing purposes. There seems to be no trouble.
After that the computer still idles.
I'll change the boot priorities next.


edit #2:
After switching fast boot and silent boot back on I deactivated the CD/DVD drives to be used for booting.
However the computer still seems to try to read from the cd-burner driver (which is deactivated for boot-up aswell).
Now I think that might make all the trouble and will unplug it and try booting again.
Thanks again for your help!


edit #3:
Damn it. That was not the source of the problem.
Will look through the BIOS settings again and throw a look at the bios message screen etc.