I ran all the virus scans and checks I could think of. No impact. Finally noticed that my DNS servers on this old WinXP computer were being set to 188.8.131.52 and 184.108.40.206. When I manually set them to those required by OpenDns 208. 67.222.222 and 220.127.116.11, I ended up with all four DNS servers listed with the unexpected 216.146.x.x ones listed as 1st and 2nd priority. Hmm.
I could see the settings by clicking Start, Run, typing CMD then ENTER and at the resulting black terminal window typing > ipconfig /all .. set my brain on stun.
Finally realized that these settings were coming from a 'feature' on the DynDNS Updater panel called 'Dyn Internet Guide' which was checked on by default. When I unchecked this feature and saved the new settings, the IP address was reported correctly on both client panels. Yay. That was easy (only a week or so of hard thinking to resolve!).