iTunes 11 has a new search feature that is enabled by default. Whereas the old function narrowed down the list of tracks displayed in the main window the new feature generates a seperate results window from which you can select Albums, Artists or Songs to either play immediately via the new Up Next feature or add to any of your playlists. While it might be possible to get used to the new version for playback, the original behaviour is both familiar and more useful for library management tasks. Thankfully you can restore the old behaviour by clicking the search magnifying glass and unticking the label Search Entire Library.
New search behaviour
Old search behaviour
You may also find it helps to restore the menu bar with ctrl-b and enable the side bar with ctrl-s. The same shortcuts will hide these features again. There is also a status bar that can be enabled with ctrl-/ but it seems a bug prevents it being turned off again under Window XP.
iTunes 11 with side, menu & status bars enabled
If you still want to roll back to iTunes 10.7 first download a copy of the installer from here, uninstall iTunes and suppporting software, i.e. Apple Application Support & Apple Mobile Device Support. Reboot. Restore the pre-upgrade version of your library database as per the diagram below, then install iTunes 10.7.
See iTunes Folder Watch for a tool to scan the media folder and catch up with any changes made since the backup file was created.
If you don't already do so I would also advise you backup the whole library on a regular basis.
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