Thank you for moving the question to the correct forum, and for providing the link with the correct procedure for the driver clean and installation.
I followed the procedure by uninstalling "everyting AMD", rebooting, running the driver sweeper to remove all video driver files, rebooting. Then I downloaded the AMD drivers from the Dell website by service tag again (in my case it's a dell install package R259285 containing among others "ATI Setup" and "InstallManagerApp" application).
After installing the driver and rebooting, the system freezes again at 'starting windows'. I can see the windows flag animation moving a short while untill it freezes, I assume that's when the ATI drivers are loaded.
Is there anything else I can try to fix this at a software level? Or is it possible that there is a hardware error that only manifests itself when the ATI drivers are installed? Are there maybe some generic drivers that allow to use at least full hd resolution in windows?