If these instructions don't make sense to you then don't bother trying it. Read them before doing anything and make sure you back up. This forum is users only; you can't go to apple and complain about it if you mess something up.
Don't credit me for this, I got the help today from Rustyintegrale and Drew Reece. I'm just putting it in one place.
1. Go to Music > iTunes > Previous iTunes Libraries and look for the most recently dated file ending with ".itl " (see photo). Copy that file on to your desktop. You will need this later.
2. You need the itunes disk image. Go to https://discussions.apple.com/thread/4555356?start=945&tstart=60
the second to last post by Rustyintegrale is a direct download of 10.7 He deserves the credit for this fix.
3. You need to download and install Pacifist. Go to http://www.charlessoft.com and dowload the disk image. Install it in to your applications folder.
4.Open the 10.7 disk image and drag the "Install itunes" image in to your desktop. Eject the disk image.
5. Quit iTunes. Open Pacifist and drop the "install itunes" image on to the Pacifist icon in your dock.
6. In the window that opens up select the top line "Content of install itunes" and select Install up in the top left hand corner. The installation will begin. When a dialogue box opens up select replace and check the box to not ask again. If another dialogue box opens up, do the same thing.
7. Once finished, close Pacifist. From finder go to Music folder > iTunes folder
Delete the .xml file
Delete the .itl file
Drag the .itl file you saved to your desktop earlier in to the iTunes folder (the folder you are currently in). Edit it's name so that it just reads "iTunes Library.itl" This step is replacing your library data from before the upgrade to 11.
8. Go to disk Utility and repair permissions then re-boot your mac. Open itunes Job done.