You do have the folder full of MP3s shared on the Windows box, don't you? No password or anything, full access rights and so on? You don't mention which Windows version it uses, so I can't give you any direct help there, but if it ain't shared, it won't be available.
Now, how are you trying to access the Windows volume? Are you using Samba or something else? Samba should be the first choice; it's very powerful, and (really the best part!) it comes with OS X. Unfortunately, it's not trivial to just say "open the Samba control panel and do..." Mainly 'cause there's no such animal-most of Samba is configured through the Terminal. There is, however, plenty of help, primarily online, for configuring Samba. Browse through this forum for a while and you'll find dozens of entries for how to share a Windows volume with Samba.