There is one little trick you need to remember if you are migrating VHDs with Vista/Server 2008 that was previously created on Virtual PC or Virtual Server 2005.
Enable Detect HAL!
How to Enable Detect HAL?
- Start the virtual machine and Run Msconfig.exe.
- In the System Configuration dialog box, click the Boot tab, and then click Advanced Options.
- In the BOOT Advanced Options dialog box, click to select the Detect HAL check box, and then click OK.
- Click Yes to restart the virtual machine.
- Clear the Detect HAL check box after reboot (it takes longer to boot).
Source: KB 954282 - The VMBus device does not load on a virtual machine that is running on a Windows Server 2008-based computer that has Hyper-V installed
Read the complete post at http://blog.mreza.info/archive/2008/06/29/migrating-vista-server-2008-vms-from-virtual-pc-server-or-vmware-to-hyper-v.aspx