Some disk utilities, like Norton or Spring Cleaning have a function that will delete files that are "unused" - meaning they haven't been changed or updated in a long time. Digital Performer creates an "invisible" authorization file when you install the application and type in the key code. These utilities sometimes detect this file, consider it to be "unused", and deletes the file.

The solution to this problem is to trash the Digital Performer folder (making sure none or your personal projects are inside that folder), and perform the easy install of DP from the CD. Once you reenter the key code and installation is complete, DP will function normally again.