Did you take a look at this page? This is for Windows 7, but it should be the same for 2008 R2. You may also be able to press F8 during the install to launch cmd.exe (on some versions, I can't remember for sure with 2008 R2).
- Boot the machine using the Windows 7 DVD
- Choose Windows Repair
- start CMD
- Run Diskpart
- Type: Create vdisk file=c:\Windows7.vhd maximum=40000 type=expandable
- Type: select vdisk file=c:\Windows7.vhd
- Type: attach vdisk (in previous Betas, it was 'surface' instead of 'attach')
- Type: 'exit' (you exit Windows Repair, but Do Not Reboot)
- Install Windows 7 to this attached .VHD (From Win7 installation, you now have to 'see' another Hard Drive, you'll need to create a partition & format it, first. You might get an error/warning saying that your computer hardware might not support this disk, just ignore it..).
- Reboot, and you got it!!!!