Okay, so a quick update on this. I tried rebooting the Windows machine, unsharing/resharing the files, checking the permissions, etc. It all seemed to look okay, and my other Windows box could see it as well.
I tried moving one of the mp3 folders to a different location, and to my surprise I could in fact see the files there. Then I moved it back to the old place - they disappeared once again. So I made a new folder in the original location, and once again they showed up fine. So now I basically just have to create a new folder for the old files in the same location, I guess. It can have the same name and everything, I just delete the old one and it works fine. Weird.
Edit: One last thing. I switched from Simple to Advanced sharing in XP, unshared/reshared, and this finally seemed to fix things. So, to summarize: Windows' fault. Easy fix, though.