I installed Windows Server 2008 x64 with Hyper-V Beta 1 on an HP Desktop and installed the Hyper-V role from Server Manager. Everything worked fine and the machine rebooted properly after installation. The role was available but the Virtual Machine Management service refused to start. It threw the following error: "Error 87: The parameter is incorrect".

The reason for the error seems to be that you need to install Windows Server 2008 using the US English locale. I chose Dutch - Belgium during the installation and that results in the error above. The solution came from here.