Windows8 update problems

Hello everyone,
to update from win7 to win8 I started the "windows8 upgrade assistant" for a compatibility check.
It tells me the computer is incompatible. A secure start is not possible with this firmware and is not supported by windows 8. This might be the reason that the upgrade to win8 defaults when the new vm tries to start.

My config: iMac OSX 10.8.2 2.7 Ghz Core i5, Parallels Desktop 8.0.18314 all updates included.

btw: I can do a clean installment of win8 on this computer with no problems, but then I have to re-install all my programs and folders which I don not wish.

Any suggestions about the incompatibility?
Thanks in advance.
Claus-Peter