Hi
One thing that you might look into is using EasyBCD to create a new boot partition on the same drive that has Windows installed on it.
It would help to see the whole Disk Management window like this...
The above would show what drive the partitions are on.
Select the drive that Windows is really on for the new entry.
If you can get it to boot from the drive that Windows is on, you should be able to backup the other partition and then format it and combine it with drive D:\.
At that point I think you can just rename it Drive C:\.
I'm not sure if there is any constraint on naming the Windows drive, but I've never had a problem naming other drives to whatever letter I wanted to.
I just checked and it didn't tell my I couldn't rename my C:\ drive.
Windows isn't really that hung up on which drive is which, if you dual boot it will call whichever drive you boot to Drive C:\ switching back and forth all the time.
It's free...
EasyBCD If you do this, be absolutely sure that the computer is booting to the D:\ drive before you remove anything from the drive that is now listed as C:\. EasyBCD should give you a boot menu after you create a second boot sector, (Don't check skip boot menu) and let you decide which disk you want to use to boot.
In any case it would be a really good idea to back up all your data before you do anything at all.
Mike