When booting Windows 2008 R2 you see this screen :
_________________________
Windows failed to start. A recent hardware or software change might be the cause. To fix the problem:
1. Insert the Windows 2008 R2 DVD and reboot.
2. Press a key when you see “Press any key to boot from CD or DVD…”
3. Select Next > Repair your computer
4. Select Next > Command Prompt
Type:
bcdedit to see if the values for Windows Boot Manager > Device and Windows Boot Loader > Device and osdevice are set to unknown
If they are set to unknown, run these three commands to correct the settings, and reboot:
bcdedit /set {default} device partition=c:
bcdedit /set {default} osdevice partition=c:
bcdedit /set {bootmgr} device partition=c:
or
cd into X:\sources\recovery and run StartRep.exe to launch a quick automated startup repair utility which corrects boot environment values.