I had this same problem but now can listen to music and even watch DVD quality movies off my comp on my xbox.
When you look through the config.xml (xbmp folder) there is a thing about SMB network.
I'll incude the code a bit later on how to use this but first I'll explain some stuff.
I originally had problems with this and wanted it bad but i got help and now its great. You need to make sure you have shared some folders on your windows system (eg, ur C: or suttin, where your music is), xbmp will use this to navigate ur pc.
in the config, find something like this, it may just be blank.....
<share>
<name>Network SMB D:</name>
<url>smb://guest:****@mshome/****/MP3'S (D)</url>
<cachesize>256</cachesize>
</share>
Its sorta simple, i've just put stars over my pw and comp name for security.
All you need to do is change the following for ur network etc.....
<url>smb://(comp logon name):(password)@(workgroup)/(comp name)/(shared folder name)/<url>
Its all pretty simple, I hope this helps, its great having everything off ur comp working on the xbox.
I have 4,900 mp3's so it takes a while to load up. If it just comes up blank then u've got a setting wrong but if it pauses, its usually loading.