If you downgrade your iTunes, you may get this message when you start the program ITunes Library.itl cannot be read because it was created by a newer version of iTunes.
to fix this:
1- Open My-documents in winxp
or Libraries in windows 7.
2- Open My Music in winxp
or Music in windows 7.
3- Open iTunes folder.
4- Rename iTunes Library.itl file to any other name or to iTunes Library.itl.old.
5- Open Previous iTunes Libraries folder .
6- Copy the newest file or any file you like. it will looks something like iTunes Library 2010-09-06.itl
7- Go back to iTunes folder and paste the copied file.
8- Rename it from iTunes Library 2010-09-06.itl to iTunes Library.itl