Background forced to black even with Vista theme

Trying to maintain my mother's PC. After an update (unclear if it was
Symantec or Windows) Vista won't allow anything but solid backgrounds.
Graphics settings are 32 bit and the latest Nvidia drivers are installed. It
previews everything correctly but it seems to ignore any wallpaper background
other than solid colors. I tried the "policy" registry fix and that didn't
have any effect on the issue. I can't find any setting that has any effect
on this. Worked normally for several months before this update.