To implement command completion requires using the registry editor, RegEdit, which when misused can render a computer or an account useless. So, follow this at your own risk.
- Run RegEdit (Start > Run, type in regedit in the dialog box).
- Go to the folder HKEY_CURRENT_USER->Software->Microsoft->Command Processor.
- Double click the key value CompletionChar.
- Change the Value Data value to the ASCII value of the key you wish to use for command completion. Not all keys work. The recommended value is 9 decimal which is <TAB>.
- Quit RegEdit.
Reference
Enable Command Completion (Windows NT/2000/XP) at Registry Guide for Windows.