When I enter the product number when starting DVD architect, I get an error message saying "It is already being used by another product." Please tell me how to avoid this.
Perhaps the specific serial number for DVD Architect is not being used for the activation process.
The best way to check the serial number is to go to the My Products listing on your MAGIX My Account and double-check the serial number against the DVD Architect entry.
If you don't have a MAGIX account and your DVD Architect was a Sony era product, you can go to:
https://www.sonycreativesoftware.com/
... login and go to My Account and then My Software and the serial number should be there.