Windows XP vs Norton Firewall
I have to give great credit to Symantec. Although it was complicated because I couldn't just open a book and get referred to page 22 or 300 or whatever, they did walk me through the process of properly getting rid of the all the Norton stuff, new and old, and reinstalling.
I say it's a nuisance, because each time they refer you to a new step it's apt to include a further referral to some other document altogether, and you wind up with a heck of a lot of printouts. My printer was working overtime on this. But that's computers, I guess.
However, things seem to be back on track, and I suspect the problem wasn't with XP at all, but with something I accidentally deleted while checking out a list of stuff, dubious and otherwise, that came up on a list of Ad-Aware or Spybot finds.