EDIT: adjust these instructions to your situation if you need a walk through i can rewrite them.
1.
Make a full system backup of the computer and system state.
2.
Log on as an Administrator.
3.
Start Regedt32.exe (or Regedit.exe in Windows XP).
4.
Go to the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
5.
Click MountedDevices.
6.
On the Security menu, click Permissions.
7.
Check to make sure Administrators have full control. Change this back when you are finished with these steps.
8.
Quit Regedt32.exe, and then start Regedit.exe.
9.
Go to the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
10.
Find the drive letter you want to change to (new). Look for "\DosDevices\C:".
11.
Right-click \DosDevices\C:, and then click Rename. In Windows 2000 you must use Regedit instead of Regedt32 to rename this registry key.
12.
Rename it to an unused drive letter "\DosDevices\Z:". (This will free up drive letter C: to be used later.)
13.
Find the drive letter you want changed. Look for "\DosDevices\D:".
14.
Right-click \DosDevices\D:, and then click Rename.
15.
Rename it to the appropriate (new) drive letter "\DosDevices\C:".
16.
Click the value for \DosDevices\Z:, click Rename, and then name it back to "\DosDevices\D:".
17.
Quit Regedit, and then start Regedt32 (not required in Windows XP).
18.
Change the permissions back to the previous setting for Administrators (this should probably be Read Only).
19.
Restart the computer.