Change the System/Boot Drive Letter
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 (http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows

1. Make a full system backup of the computer and system state.
2. Log on as an Administrator.
3. Start Regedt32.exe.
4. Go to the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
5. Click MountedDevices.
6. On the Security menu, click Permissions.
7. Verify that Administrators have full control. Change this back when you are finished with these steps.
8. Quit Regedt32.exe, and then start Regedit.exe.
9. Locate 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.

Note You must use Regedit instead of Regedt32 to rename this registry key.
12. Rename it to an unused drive letter "\DosDevices\Z:".

This frees up drive letter C.
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.
18. Change the permissions back to the previous setting for Administrators (this should probably be Read Only).
19. Restart the computer.