JonD
Check your LAN settings (Control Panel - Internet Options - Connections). Make sure it says "Auto detect" instead of a proxy (Unless, of course, you've set up a proxy).
Thanks. I did check that setting and it is on. Meanwhile, it has begun working normally for me again. The only thing I did in the hour or so between when it was failing and when it was working was I updated the firmware on my router. It was only a minor update because I had updated it a month ago. But that did force a reboot of the router, of course. I had rebooted the router (a complete power off cycle of everything -- router, switch, PC, and Comcast cable modem) earlier in the day and that had not changed anything.
My TRACERT commands implied routing problems inside the MSN facility, but I may not be interpreting that correctly.
So I don't know what changed, but it is now working normally.