Meanwhile, I found out Vista is the last officially supported OS for this computer. As I also have a Vista license I installed it and it's running fine.
I think I'll try Windows 7 again soon. I think it would now work.
I adjusted BIOS settings and I reattached the SATA cable. It seems it did the trick.
Thank you all for your help!
Thanks for these steps! As I'll try installing Windows 7 again, I'll try these.