I would suggest two things.
1. view your config-inc.php file and make sure the correct vbulletin license number is entered from their config file
2. View with phpmyadmin view settings 171-173 in the settings table and set to match cookie settings of vbulletin. Most likely as Zach stated your cookies are not set correctly. You would either need to do this or reinstall with the correct info if your not familiar with phpmyadmin