To finalize this thread:
I have installed the Release Candidate of Windows Server 2008 R2 which finished without a problem. But after I added the Hyper-V role I found the following entry in the event protocol:
Error starting Hyper-V. The processor 0x0 does not provide the functionality, which is necessary to run Hyper-V. (endpoint 0x1, register 0x3, necessary function 0x20000, supported function 0x0)
(my translation from german)
Source: Hyper-V Hypervisor
Event ID: 48
So, I'm giving up. :(