It is a problem a lot of Vista and nintendo USB dongle users had, including me, and no one seems to know a solution other than uninstalling and reinstalling the drivers... which is annoying because you have to redo it everytime you want to play.
If you work your 52103 error further (go in your control panel, in the networking menu, connection managing, you can modify something with the new icon the dongle is making use of... you can play with that and) you will get a 52003 error... it is doing the same thing to your DS in the connection test with the indicator going from 3 to 0 bar and then "error".
It isn't because of firewalls and antiviruses...
I've tried so many things... I frickin hate it.
Do you have a router?