Changing back the drive letter from D:\ to C:\ could solve the problem.
As long as Windows is up and running, you won't get the allowance to change the drive letter of the disc where Windows is running on.
You need a tool which will do the trick in DOS mode. I think I remember a tool called
Partition Magic is able to serve you in your situation.
As an alternative, Format C:\ or Format D:\ after an external backup will lead you to a virgin setup.
Before installing the OS, unplug the disc you don't want your OS on. That way you won't get in trouble with the drive letters. Plug it in after the new installation and nothing will be changed by accident.
BUT: Be careful with Partition Magic, b a c k u p y o u r d a t a f i r s t