For the most part, this is not recommended, especially if the drive letter is the same as when Windows was installed. The only time that you may want to do this is when the drive letters get changed without any user intervention. This may happen when you break a mirror volume or there is a drive configuration change. This should be a rare occurrence and you should change the drive letters back to match the initial installation.

To change or swap drive letters on volumes that cannot otherwise be changed using the Disk Management snap-in, use the following steps:

Note: In these steps, drive D refers to the (wrong) drive letter assigned to a volume, and drive C refers to the (new) drive letter you want to change to, or to assign to the volume.

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

1. Click MountedDevices.
2. On the Security menu, click Permissions.
3. Check to make sure Administrators have full control. Change this back when you are finished with these steps.
4. Quit Regedt32.exe, and then start Regedit.exe.
5. Go to the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices

1. Find the drive letter you want to change to (new). Look for "\DosDevices\C:".
2. Right-click \DosDevices\C:, and then click Rename. In Windows 2000 you must use Regedit instead of Regedt32 to rename this registry key.
3. Rename it to an unused drive letter "\DosDevices\Z:". (This will free up drive letter C: to be used later.)
4. Find the drive letter you want changed. Look for "\DosDevices\D:".
5. Right-click \DosDevices\D:, and then click Rename.
6. Rename it to the appropriate (new) drive letter "\DosDevices\C:".
7. Click the value for \DosDevices\Z:, click Rename, and then name it back to "\DosDevices\D:".
8. Quit Regedit, and then start Regedt32 (not required in Windows XP).
9. Change the permissions back to the previous setting for Administrators (this should probably be Read Only).
10. Restart the computer.

Links

Change the System/Boot Drive Letter in Windows - 223188

Unable to Log on if the Boot Partition Drive Letter Has Changed - 249321