After 4 full days I finally found a solution!
Anyone who wants to change their RAID mode back to IDE mode in the BIOS, the standard procedure of changing it in the BIOS and a reboot won't work. You will have a Blue Screen of Death (BSOD) all the way.
You HAVE TO EDIT THE REGISTRY. That is the solution! To get into the Registry on a working Windows system, press Windows+R to bring up the Run dialog. Type regedit
and press OK.
Follow this procedure from this link, which states to change these values under HKEY_LOCAL_MACHINE\SYSTEM\Currentcontrolset\services\
:
adpahci
key, the start value from 3 to 0
atapi
key the start value from 3 to 0