Apple released an update for iTunes and other of their software on 9th Sept. Many folk who downloaded these updates to PCs have had problems of all sorts since, mainly not being able to launch iTunes at all, but including change of language, loss of libraries, inability to access various hardware in their computers,and there are plenty of angry messages on the Apple discussion forum. I myself could not get into iTunesa ta all as I kept getting a missing file message, as well as "can't install because can't remove older version of "Bonjour" Two days ago Apple released a "bug fix" version 9.0.1, but this still did not work for me, until I found the following solution:
Having downloaded the Wndows Installer Clean-up Utility, I was then able to remove the old Bonjour (adieu, not au revoir!). I also uninstalled Quicktime. Then downloaded iTunes 9.0.1 and finally everything's working again and all my music is there.