1. Start -> Run -> Notepad.

2. Paste the following:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Directory\shell\Notepad]
@="Notepad"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Directory\shell\Notepad\command]
@="C:\\Windows\\notepad"

3. Save it as "notepad.reg"

4. Open it, it will ask you if you want to add the data, select yes.