For Magic DVD Ripper, when you have gotten a confirmation that your registration code has been accepted, sometimes the application will stop working and ask you for the code repeatedly. If you do not have a valid copy of MDR, do not attempt this.
To reset the application after a full reinstall, delete the following key in your registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ALP\option
You will usually see this error: "Register Code is not correct! We strongly recommend you copy/paste it."
For Windows XP x64:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\ALP\option
This fix applies (at least) to v5.2.1.