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When you turned on media sharing in Windows Media player, did you enable the DSM-520 as an authorized device?
Yes. I am able to ping the DSM-520 and response time is very good.
Have you tried removing the XP machine from the network? Do you load the D-Link server (or TVersity, Twonky, et.al.) before scanning for servers? I don't think Vista is your problem, but there is something set up incorrectly. Try going thru the setup procedure again, perhaps after a hard reset on the 520 to see if you can identify a server on your Vista machine.
Have you tried to also use Active-TV? since you have firmware version 1.05 you can use Active-TV as your server.