Copy your iTunes Media folder from the Windows drive to the Mac drive. Eject the Windows drive when this has finished.
Copy all the files and folders from your user folder on the PC to the ~/Music/iTunes folder on the Mac. (~ is shorthand for your user folder, the one with the house icon and your user name). If there is no iTunes folder, create it, and then copy the files. This includes the .itl file, the .xml file (if there is one), the Album Artwork folder, and any other files.
Via David Anders