You can use the built in Samba support. I only share music too and this is what I do.
First you will need to share a folder on your computer. For example lets say you called the folder mp3.
Okay now that you have the share set up on your PC you need to edit the xboxmediacenter.xml file and add a line similar to this under you music shares section.
<bookmark>
<name>MP3</name>
<path>smb://username:password@workgroup/mp3</path>
</bookmark>
Username = the username that you are logged into windows with
password = your password
workgroup = the workgroup name that your computer is part of
Reboot xbmc and you should now have a folder called mp3 under the my music section.
Regards,
KP
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