Just installed the new Windows Server 2008 RC1 with Hyper-V beta and, after adding the Hyper-V role, created a new virtual machine. Trying to start this machine gave a message that the hypervisor was not started. This is not uncommon but is almost always the result of either having a host CPU that doesn't support hardware-assisted virtualization or hardware-assisted data execution protection (DEP) or having these features turned off in the BIOS.
Not so in my case. The hypervisor kept failing to start giving some ominous error that its internal logic had failed. After some poking around I noticed that I had a mass storage controller that did not have a driver installed and was not functioning. It was my SATA controller and since my system was operating in IDE mode was essentially not being used. Still, I disabled the controller and restarted the system.
This time the hypervisor started up correctly and I was able to install the VM. Just a little FYI. If your hypervisor fails to start, make sure your host hardware is all setup and working correctly.