But, when I wanted to do just that, there was a problem. I got strange errors (Error 32000) that stated that the virtual guest services couldn't be installed and in the event log of the guest I got errors that stated that a SCSI device "disappeared from the system". Wait, what?
This is the point I found out that my VM still had the old VM additions of Virtual Server installed. It could be the problem. Maybe there's some interference between VM additions and virtual guest services. But when you try to remove the VM addtions, you get errors that the guest OS is not a supported OS. That is a strange error, considering that the additions are in fact already installed and that you just want to remove 'em.
Well, I did some Googling and found the solution. The trick was to mount the VHD's in a Virtual PC VM. This gives you the opportunity to uninstall the VM Additions without errors. The resulted VHD's can then be remounted on the Hyper-V VM and, hurray(!!), the virtual guest services installed without any problems.