In Windows Virtual Server 2005 to mount a VHD as a drive you would use the VHDMount.exe program.  So to mount server1.vhd you would run the following command:

vhdmount /m /f d:\server1.vhd

The /m plugs the drive in and mounts a volume for use.  The /f mounts the volume without creating an undo disk.  That's it.  Pretty simple.  To unmount the drive you would just run the following command:

vhdmount /u d:\server1.vhd

Well, on Server 2008 R2 running Hyper-V it's even easier to mount a VHD.  Pleaese keep in mind the process I will show requires a full install of 2008 R2 (If you are running CORE you will have to use command line or powershell).

1. Open up Server Manager and expand Storage
2. Next Right-Click Disk Management and select Attach VHD.
3. Browse to the location of the VHD you would like to mount and click OK. (You can also check Read-Only to mount the drive in read only mode.)
   
4. The VHD will mount as a drive using the next available drive letter.
5. To Unmount the VHD simply Right-Click on the Disk under Disk Management and select Detach VHD.
   
   
   

That's it!  Pretty easy.