can you connect to the PC from the Mac, using the Go menu in Finder?
If not, go to System Preferences, Sharing and turn on File Sharing and Windows File sharing.
On the PC, create a folder, My Documents will do, and share it. Make sure you put all your music inside this folder. The music folder will do. If you have any Firewall software running, you will need to configure it to allow machines on your network access to each other.
Or just turn it off for a while.
From the Mac, using the Go > Network menu option, you should be able to see the PC. Connect to it using the name and password of the PC user. (Not the Mac username and password)
Once connected, open the shared My Documents folder and drag all of you music from the PC to the Music folder on the Mac.
Any tracks that you find are missing can be dealt with by launching the PC iTunes and exporting them to the Music folder. Then just drag them over to the Mac.
Try that and let us know how you get on.
P