I just got my eXplorist 400 today and I am having the EXACT same problem!
Some notes:
I tried switching the direction the cable was screwed in - when it is facing up over the top of the device, Windows detects it but says no drivers are installed. When I tried to get it to load the drivers on the CD that came with it, it gave me the same error.
Conversely, when I attached the cable in the recommended direction - down over the battery case - the gps device itself goes into File Transfer Mode, but Windows doesn't find or install the device at all.
I have also tried "screwing it in tightly" as recommended, but to no avail.
O/S is Win2k, SP4
My mp3 player, camera, and flash drive work fine so I know it is not a problem with my USB setup.
Is this just a common problem with several models of Magellan's? There does not appear to be a way in the 400 to manually enter waypoints to use the GOTO feature either unless I am overlooking an obscure menu feature... So if I can't get it connected to my PC I can't go geocaching at all...
I almost bought the less expensive 210, but I may switch to a Garmin if it looks like I can't resolve this - a pity too, since the Geocaching feature and the GOTO button seemed like they would really be useful...
-Scriv