Windows 7 changes main partition drive letter from C to D?

I have noticed this few times already on different computers, manufacturers; but all running some version of Windows 7.

Customer calls that PC is not booting up but it keeps going into Win7 repair options, over and over again.

I come over, try to do build in system repair (fail), system restore (fail) and I even try repairing from the disk, but with no results. I can not even go to safe mode before it dumps me back to system repair options screen.

Every time I notice that system repair stated that OS is located on D drive not C drive. Now, I can not confirm that but I am 99% sure that their OS was on C drive.

What could be causing it and/or how would I get around this problem without having to reinstall the PC?

Thanks