Uwe,
I have been using Win 8 since the first developer release. I currently have 5 computers running under the RTM version and had no problems installing on any of them.
Yesterday I reinstalled on a fairly new laptop. I wanted to put a non-RTM license on it in case I want to sell it. I reverted the computer to the Win 7 install, did the "purchase" and download, and all went fine.
Here are some tips. First, the program checks to see what is NOT compatible, and then will uninstall those programs. I did NOT reboot after each uninstall. I just answered "Later" on those questions. Then I completed all of the uninstalls. The last option IS a Reboot, and I let it do it at that time.
The update installed fine ( but remember that Microsoft can not imagine every possible program or options that are already installed ). Then I went to the computer manufacturers website and got their updated drivers for Windows 8. In this case, its Samsung and they have a utility you install that automatically gets all the new programs and drivers for the individual computer, downloads them, and installs them. It worked fine.
I have done these installs 6 times myself, and I have several friends who are not "techies" who have already done the update.
I'd suggest you try again, and this time do not reboot after each uninstall. Also, if you have some startup scripts, disable them until after you do the update, then you can likely put them back in place.
If you think about Win 8 as a more powerful Win 7 Desktop with a whole new set of opportunities ( RT ), you will realize why some of us who have used it for awhile are very pleased.
Tim