So you have used iTunes on windows and have an iTunes library on their?
I am just guessing here, so I could be wrong. If there are no WMA files, and your iTunes only contains file types that are recognized on a Mac, you should be able to move the iTunes folder over to the Mac.
Just connect over the network, and replace the Mac's folder called 'iTunes' in the Music folder with the one from the PC.
Then when starting iTunes on the mac, right click and 'choose library'. If it worked correctly, the process of loading the library will take some time depending on how many songs you have.
Then just elect to sync the iPod with the newly moved iTunes library. Remember that this will erase the iPod.
This process worked fro me when moving an iTunes library from Mac-to-Mac. Not really sure if it will work PC-to-Mac