Copy the files over the network. Enable file sharing by going through the network wizard (in the Network Places or Network connections folder) and setting your workgroup (I recommend WORKGROUP, the Mac OS default workgroup) and clicking the option for file and printer sharing. Right click on the My Music folder and click the sharing option and enable file sharing for that folder. The PC's music will then be accessible on the Mac. Drag it into your iTunes folder into the main Library window in iTunes from the Mac.
You can export your library file by using the option under the file menu on the PC and import it on the Mac in order to recover your playlists.