If the software is already installed (e.g., as a "Test Drive" or demo), you can often enter a new validation code without reinstalling by using the command within the application menu. Helpful Documentation
Are you trying to for an old machine, or QuarkXPress 5.0 Serial Number Lookup & Activation Code Help quarkxpress 50 product validation code
If you are installing a full version or an upgrade, you will be prompted for this code. Check Original Documentation: If the software is already installed (e
: Printed on the installation card or the original invoice that accompanied the software packaging. as a "Test Drive" or demo)
Version 5.0 was the last to work on Windows 3.x (with Win32s) and was notorious for not natively supporting Mac OS X at launch, which may cause errors on modern operating systems.