Weird 15" LCD display problem

[Jim Robertson]

My son has a 450 MHz Sawtooth G4 with the standard AGP video card (Rage 128
Pro). Yesterday his monitor image suddenly inverted (looks like a b/w or
color negative, depending on how he sets preferences for the monitor). This
must be a software issue, because the image is normal if he logs out and
remains normal if another user logs in (OS X 10.2.5). I did a quick search
in the support section of Apple's website and couldn't find anything that
addresses this. Playing with the brightness settings for the monitor works
but doesn't revert the image to normal when he's the logged-in user.

Is this a preferences corruption of some sort?

One possible clue: he says he "might" have inadvertently tapped one of the
keyboard's F-keys just before this happened. However, we don't have any
add-on software loaded that does things with the F-keys.

Thanks so much for any/all suggestions.

Jim Robertson
--