Here's the link to retrieve your lost registration key http://support.apple.com/kb/HT2376
What version of QT did you have on XP? If you had QT version 6.X pro then you would have to purchase a new QT pro key for QT 7. As Apple releases complete new versions of QT they require a new pro key, updates to a version do not. In other words the pro key for QT 6 does not work with QT 7.
And if you were using QT pro 6.X you cannot revert back to that version. Because QT 6 doesn't support windows 7
Jacumba, since I purchased QT Pro this past September I'm using version 7.
I followed the link you posted. It said for me to visit the Apple Online Store and go to the Your Account page and view my order history. So I did that.
This is helpful insofar as I can see a record of what I ordered. But there appears to be no link where I can download the QT Pro application again.
Am I missing something here? Shouldn't there be a "Download" link or button on this page?
QuickTime Pro is not a separate application. All you get is a key that unlocks the editing features of the normal, free version of QuickTime Player. Just download and install QuickTime and then go to the Preferences under the Edit menu in QuickTime Player and select "Register". Enter in your user name as reflected in the license email and the Pro key.