You can import your library details, including ratings and play counts, by copying your iTunes Library.xml file to your new computer (say, the desktop), then from iTunes do File > Library > Import Playlist and select the xml file. Make sure you have all of your music in the iTunes Music folder first or the import will just ignore the items in the xml file.
Julien: I'm not sure where that Level 2 support guy came from, but this isn't difficult to do.
Edit: There's even a knowledgebase article about it -
http://support.apple.com/kb/HT1451