I had the same problem, I was unable to see or map the dns-323 drives, even my Router i count not find.
It was only when I used Wi-Fi I had the problem, when I used cable to the router, I had no problem..
Im running Vista+SP1 with all updatets.
I had no problem what so ever with XP...
I solved my problem, but turning on uPnP in the Router! and rebootet everything (Computer, Router & NAS)
also take a look here, I also posted my fix there : http://wiki.dns323.info/information:known_issues