I got the same message on my Dell XPS but I was persistant and got 11 installed. I lost around 1400 songs, all audiobooks, around 50 movies and all my playlists. I literally worked on iTunes all day last Sunday and, after several calls to Apple customer service, I killed 11 and reinstalled the last version of 10. iTunes wouldn't work at all as 11 had changed some of the .itl files. Finally, I got an Apple tech to e-mail the instructions on killing 11 and reinstalling 10 (it took over 2 hours to do this). After that, I got everything back except my playlists.
I will install 11 sometime but not in the near future. By the way, even if I could have gotten 11 up and running, I didn't care for the new layout. I found it slow and cumbersome.