This issue drove me crazy for days. You would think the obvious solution would be to go through the Edit Menu, to Preferences, then Registration and simply remove the name and key. It appears that it is gone until you go back into the settings and the name and key is still there.
Here is what I did to resolve the issue:
If you are running Windows 7:
1) Go to Windows Explorer. Click ‘Tools’ then ‘Folder Options’
2) Click ‘View’ Tab
3) Click radio button, ‘Show hidden files, folders, and drives
4) Close Windows Explorer
5) Re-open Windows Explorer
6) Navigate to folder ‘C:ProgramDataApple ComputerQuickTime’
7) Delete the file, ‘com.apple.QuickTime.plist’
Note: This file is readable in notepad and contains the registration name and serial key.
8) Reopen Quicktime Pro and the registration is now blank