The nightmare continued for a bit, but now it's working.

Because once in Windows 7, my USB3.0 ports didn't work again. So no way of installing the drivers. So I allocated the 2nd internal HDD and copied the drivers from the DVD to a stick from my other PC.

Then I booted Windows again from the stick, and chose Repair Windows. There I have the opportunity to run cmd. Then I copied with xcopy /e all the contents from the stick to the internal HDD. After rebooting I was able to install the drivers from the HDD. Did USB3.0 and LAN first, and it works fine now.

What a journey