When I try to browse to a shared resource and I can't drill down into it, I normally am forced to reboot the shared resource. I know that sounds absolutely stupid, but it works. I have a Windows 7 PC with both internal and external drives shared with Boxee, and a reboot will normally take care of any share issues I encounter. Ronald can probably back me up as I attribute this to an unusual implementation of SMB by Boxee. It's not an issue with the PC as all other PCs on the network can browse the shares normally.
Hope this helps.
Router: ASUS rt-n56u (Black Diamond)
Server: Intel Quad Core 8200, Radeon 3450, 4GB RAM, Windows 7 Ultimate, Wired Gigabit
Boxee Box: latest firmware, Wired Ethernet
Connectivity: Netgear AV500 Powerline Adapters