Resolution (found in Microsoft KB 691661):
When you enable the Hyper-V role in Windows Server 2008, do not install the drivers for high performance accelerated graphics adapters. This behavior will not occur when you use the Vga.sys or Vgapnp.sys generic video drivers that are included with Windows Server 2008. To revert to the generic video driver, you can uninstall any high performance vendor-specific video driver.
(see also what Virtual PC Guy has to say about this)