You can convert D to C by copying all the files from the existing C drive to the D drive, making an Emergency Recovery Disk ( you use the Win2K Backup program for this ), and then deleting the C partition and using Powerquest's Server Magic or equivalent to resize the D drive. When you reboot you can use the ERD to fix any boot-up issues.
But...all the path pointers for your Win2K install ( as well as those of any installed applications ), will point to the D drive for system files instead of the C drive. There are utilities available that are supposed to fix this issue, but I've not had much luck with them.
So...The quickest/easiest way may be to just backup all your data and rebuild the server with a repartitioned/reformatted drive.