Manually: regedit and navigate to
HKEY_LOCAL_MACHINESOFTWAREClasses*Shell
add the key "notepad"
it's default value should be "Open with Notepad"
now, under "notepad", add the key "Command"
it's default value should be (with the quotes) "C:WindowsSystem32Notepad.exe" "%1"

ok, exit regedit and go right click on ANY file...
your new option: Open with Notepad

I know this one works on XP and 2000, you can PROBABLY enter it manually on windows 9x, but i have not tested that