On the PCs, go into the Network control panel and enable sharing. Once you've done that, find the drive or directory they want to share, right click it and choose sharing. Now, I think the easiest way to transfer music is for you to connect to their computer, send all of your music to them, and then grab theirs. They can exchange music between themselves. It's kinda tricky to share with OS X, because you can't easily share specific directories, and I've never been able to get a Windows 98 computer to connect to my Mac. If they're running Win2K or XP they should be able to.
Brad