I think I found your problem. This is from QuickTime 5's FAQ on Apple's site.
tWill my QuickTime 3 or 4 Pro key work with QuickTime 5?
tQuickTime Pro keys bought after October 12, 2000, will work in QuickTime 4, QuickTime 5 Public Preview, and QuickTime 5, including QuickTime 5 for Mac OS X. Keys bought before this date will work in QuickTime 4 and QuickTime 5 Public Preview only. If your key no longer works, visit the Apple Store to purchase a new one.
So if your old key was for QuickTime 3 or 4 and was originally purchased before October 12, 2000 then you will need a new key for QuickTime 5.