1. Restart your computer and press the “F8” key on your keyboard before Windows starts up.
2. Press the “down” arrow key on your keyboard to highlight the “Safe Mode” option. Press the “Enter” key on your keyboard.

3. Select your computer with the arrow keys and then press the “Enter” key. Your computer will then start in Safe mode and you’ll be prompted to log in as a local administrator.
4. Click on the “Start” button, then click “Run.” Enter “cmd.exe” into the “Open” field and then click on the “Open” button.
5. Enter “msiexec /unregister” into the command prompt and press the “Enter” key. Enter “msiexec /regserver” into the command prompt and press the “Enter” key.
6. Type “exit” into the command prompt and press the “Enter” key. The Error 1721 will be fixed and you can remove programs using the 'Add or Remove Programs' feature.