This tool has to be used from windows. If you are reinstalling xp and have gotten to the point where it asks you for the key it's to late, the drive has been formated at this point unless you did a custom install. Plus for it to work from DOS you would have to create a boot disk to Read NTFS since most xp Installs use this format. If you used a boot disk that reads NTFS then you would still have to be able to access the registry since that is where the program gets the product ID from.