Boot into safe mode
and press CNTRL-ALT-DEL. Select the task manager and select file and run.
Enter explorer.exe. When the explorer opens see what drive letter windows is using as the system disk.
Then select regedit and navigate to HKEY_Local_Machine\System\Mounteddevices
Look for \DOS\Devices\?:
where the ? is the drive letter that the system was using. Change it to C:
First look to see if there is another \DOS\Devices\C:
, if there is, change that letter to an unused letter before
changing the system used drive letter to C:
Reboot and you should be all set.
You will also need to reset the system restore because it will have a drive C: that it can't find.