The simple way

You can change the local drive letters in Start, Control Panel, Administrative Tools, Computer Management, Storage, Disk Management.

Changing the Drive Letter of a System Partition

Changing the drive letter of the drive that contains the boot files or the system is more involved.

First of all, it is very difficult to change the drive letter of a partition with a Windows installation from that on which is was originally installed, because there are usually many references to the drive letter in the registry and in INI files, and some may even be hidden in unknown configuration files.

Therefore you should only attempt to change the drive letter of a partition with Windows back to what it originally was.

If you installed Windows XP or 2000 only recently, it is much better to fix the problem that led to the wrong drive letter, usually by removing removable drives, then install Windows again.

Please check the following Microsoft Knowlege Base article. It was written for Windows 2000, but works perfectly well for Windows XP.

Q223188 - How to Change the System/Boot Drive Letter in Windows 2000
http://support.microsoft.com/kb/223188/

If you copied an entire Windows installation and now find that your new system drive letter is no longer C:, you can use this method to change the drive letters accordingly.

Then remove the old drive or set its partition to hidden, to test whether drive C: alone is now bootable.

After this you can reinstall the old drive if you like or make its partition accessible again.

Changing the system drive letter if you cannot boot or log on

This procedure is even more complex than the preceding one. Some important details have been added by Dave Everett. Thanks, Dave!

1. Connect the problem hard disk to another computer or boot another operating system installation on the same computer, if you have one.
2. Identify the SYSTEM hive structure of your system.

   It is stored in the SYSTEM32\CONFIG folder. Its filename is just SYSTEM without any extension. Ignore files like SYSTEM.LOG, SYSTEM.ALT, SYSTEM.SAV, or the like.

3. Fire up the regedit.exe registry editor (or the old REGEDT32.EXE, if you are still running an early XP version where regedit.exe cannot do this).
4. Load the SYSTEM hive into HKEY_USERS on the helper computer.

   To do this, select the HKEY_USERS window. In other windows, like HKEY_CURRENT_USER, the Load hive ... command is greyed out and not available.

   Use the command Registry, Load hive ..., to load the SYSTEM hive temporarily into the helper computer's registry.

5. Remove the offending drive letter assignments.

   To remove the drive letters, navigate to the registry key:

   HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices

   You can then change the drive letters with the same procedure described in the Microsoft Knowledge Base article mentioned above. Example:

   • \DosDevices\C:
   • \DosDevices\D:

   In this example edit the value names as follows. Change C: to Z:, then D: to C:, then Z: to D:.

6. Unload the hive to remove it from the helper computer's registry.

   It is not necessary to save it, as the changes are made to the actual hive file.

7. Put the thus repaired system disk back into its computer and try again to boot.

   Disconnect all other drives that could snatch the desired drive letter.