|The C: drive is usually on disk 0. Is it a SATA or IDE drive?|
So I assume the phone was attached to a USB port? Try booting with the phone attached (assuming it's not attached now) and see what happens.
Are you just assuming the drive letter was reassigned or did the software give you that specific message?
Use F8 again to get to the boot menu and choose 'safe mode with command prompt'. If it loads what drive letter is showing at the command prompt--C:\> is C:, D:\> is D:, etc?
Apparently the fix for a reassigned boot drive letter is some registry editing. I found this:
But it seems if it is the registry and you can boot to command prompt successfully then maybe a system restore would work.