I don't understand... You got the Win2008 installation media right? Microsoft doesn't stop you from burning that intto ISO file... you can even install the VM from the disc directly. Just point your VPC/Virtual Server to mount the physical drive.
You can even download the pre-installed
Win2008 evaluation image VHD if your VM is only for software evaluation (60 days plus 3 chances of re-arm, so max. 240 days). So just download it and mount it if you want it this way.