some apps fail unless run as an administrator

4 month old Dell shipped with Vista Home Premium pre-installed. I set up my
account as admin, created two regular accounts, one for the kids, one for the
wife. All was well until about a month ago when Windows update hosed the OS.
Most apps would open, but fail to function properly (broken sidebar icons,
window opens, but no app runs, etc...) unless I right-clicked and chose to
run as an admin, then they ran fine. So, I restored to a point just before
the problem, and all was well, the update happened again, but without
incident. Until now...
Similar problem again. As an admin, I have no problems. But, my wife's
account just got worse and worse over the course of this week. First, the
sidebar. It opens, but instead of widgets, there icons like a broken image
icon on a web page. Media Player won't launch (see it flash in task manager
for a second, then disappear). Same for other apps, including her Thunderbird
email client. But, if I right-click, choose run as admin, the programs work
just fine. I even tried to set my wife's account as an administrator account,
but that didn't help.

Any ideas what could be causing this?