Ensure you either have guest access enabled and your file shares are marked as readable by everyone, or that you have your folders shared by a specific user account that you can enter on your Boxee Box.
In your networking settings, make sure that file sharing is enabled; this should be set by default if your network interface is set to home, but it may be disabled by accident.
If possible, browse your network using Explorer (open a Windows Explorer pane, go to the lower left corner and select Network, then choose your workgroup - which is probably 'WORKGROUP' or 'MSHOME' - make sure your machine shows up there).
Make sure the nmbd (netbios name resolution, this is part of samba) is running on your server.
If possible, validate that you can access the shares from another machine.
Ensure that either a) your shares are marked as guest accessible, or b) your shares are accessible by a user account that you can enter on your Boxee Box.