Connection refused?

Either it's the bad address, the bad firewall, or the bad sharing service. You aren't sharing via W7 Homegroup? Homegroup networking is W7 only.

The fact that the names don't work suggests that there is a firewall/router between that doesn't allow for some packets to go through. Since the machine you tested is likely wired and the N900 is definitely wireless, I'm guessing there's a wireless router there - you might want to check settings.

Try to telnet (might needs installing) to the machine via 139. That should clear a few things up.