Thanks for all the feedback here.
That was a good tip to check the UAC details -- I did so and discovered that it referred to MC23, NOT MC24. In other words, even though this started with the MC24 install, it's MC23 that's suddenly asking for the admin password on bootup.
My inclination was to solve this by just uninstalling MC23, but when I went to do this, one of the questions the uninstall routine asked was if I wanted to remove "All Media Center registry entries." Given that it said "Media Center" and not "Media Center 23," my concern is that this could potentially mess up not the current MC24 installation and maybe even the uninstall process for MC24 as well (which may be needed if things get really screwed up.) On the other hand, NOT removing all the "MC23" registry entries might lead to complications as well.
I therefore waited to uninstall MC23 until I can get confirmation on the best way to proceed. Can someone solidly confirm whether or not I can just uninstall MC23 without it having any effect the parallel MC24 installation? If not, what would the best approach be? I just want to avoid ending up in an install/uninstall hairball.
Thanks again for the help here,
Larry
UPDATE: After searching the forums a bit, I found a post saying that you could safely uninstall MC23 INCLUDING "all registry entries" without it effecting MC24. If this is not correct, please let me know -- I'll wait until tomorrow to actually go through with it.