Thanks MindoverMaster. I do appreciate your help!
I disconnected first my first cd-drive. Then my second. And then my slave harddrive. It didn't help. The computer continued to reboot. When I finally disconnected my master harddrive the boot process stopped with a message saying: "Reboot ans Select proper Boot device or Insert Boot Media in selected Boot device and press a key".
I connected the cd-drives again and inserted my Windows installation cd. Now, no message appeared, but a vertical row of H-letters.
The wattage of my PSU is 500 watts.
Here is the specification of my computer: