The following method involves some risks (as is the case with all modifications done to the registry) but if you know what you are doing I’d say your chances of messing it up are close to null.
- Log on as an Administrator.
- Start Regedt32.exe.
- Go to the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
- Click MountedDevices.
- On the Security menu, click Permissions.
- Verify that Administrators have full control. Change this back when you are finished with these steps.
- Quit Regedt32.exe, and then start Regedit.exe.
- Locate the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
- Find the drive letter you want to change to (new). Look for “\DosDevices\C:”.
- Right-click \DosDevices\C:, and then click Rename. Note You must use Regedit instead of Regedt32 to rename this registry key.
- Rename it to an unused drive letter “\DosDevices\Z:”. This frees up drive letter C.
- Find the drive letter you want changed. Look for “\DosDevices\D:”.
- Right-click \DosDevices\D:, and then click Rename.
- Rename it to the appropriate (new) drive letter “\DosDevices\C:”.
- Click the value for \DosDevices\Z:, click Rename, and then name it back to “\DosDevices\D:”.
- Quit Regedit, and then start Regedt32.
- Change the permissions back to the previous setting for Administrators (this should probably be Read Only).
- Restart the computer.
From : Overclock.net