Ok, click start right click on computer and select manage
Then in the new window click on drive management
Right click on what should be D drive that is now C and select Change Drive letter and change it to anything.
Then right click D drive, change drive letter then choose C drive.
Then right click on the drive you changed before that should be D drive and select Change Drive letter and set it to D
It will then be corrected.
I can also do these steps for you, and have made a request via Additional Services. This is optional, and if you choose to do this, I can, with your permission, remote into your computer while you watch and do all these steps (and deal with any problems that arise during them) for you. Or if you do not choose to take this service, we can continue here. This is no problem at all. Just let me know please.
Else let me know the results please.