It seems Safari 5.1 might be the problem (at least it is for me). I've managed to fix mine by uninstalling all my Apple software, deleting the Apple related folders in Application Data, Local Settings and Program Files that weren't removed by the uninstallers, rebooting, and reinstalling iTunes 10.4.0.80. The problem resurfaced after I installed Safari 5.1 again so I have repeated the removal process and I'm not reinstalling Safari for now. I only really had it for test purposes and use Google Chrome for preference.
NB: Removing all the settings folders will also remove any device backups you might have, so it is probably best to rename the Apple folders, rather than delete them, until you've had a chance to reinstall iTunes and backup your devices.
Update: It would seem Safari 5.1 or QuickTime 7.7 can trigger the problem according to this thread Mine had seemed fine after an update to QuickTime but is broken again now. Another hour of uninstalling-reinstalling to do. Grrr 😟
tt2