8 for XP -- got a new problem I'd never seen before (access denied because of credentials submitted), but got a quick email response from MS Support suggesting I check, among other things, my Local Area Connections DNS settings. I don't remember ever touching those, just the ones for my ISP connection, but once I set the LAC ones to MS's recommendations, Update went just fine.
Wandering somewhat OT, it'll be interesting to see if the change makes any difference to the extremely slow lookups for HTTPS addys I've experienced with Firefox 3.5.x.