I believe the ratings are stored in the 'iTunes 4 Music Library'
Located in something like C

\Documents and Setting\user\My Documents\My Music\iTunes
On OS X they would be in ~/users/Music/iTunes/
I have no idea if the files are compatible across platforms.
You could back them both up and try moving it from XP to X.
You might also try zipping the iTunes folder on the PC and placing it in the Music Folder on your Mac. Taht would move all music and other settings.
Hope this helps some.
T