Here are a few ideas to play with:
1. By default the AT will only look in shares that have been called
'My Music', or "Music' or 'Audio'. When you shared c:audiotron, did you
call that share one of those three names or did you change the AT's default
OS that it searches all shares regardless of name?
2. Which firewall do you have? Is it possible that it is screwing things up
even though you have disabled it?
3. Have you disabled XP's 'simple file sharing' and then after doing so
created the share permissions for atron?