>What could be causing this problem? Is it a problem with the
>display, printer driver, or application?
>
I'd bet on the video driver - confirm this by having the user temporarily switch to using the Standard VGA video driver - if the problem goes away the video driver is pretty clearly implicated.

You may find that changing the color depth (the number of discrete colors that can be selected from per pixel) will clear up the problem, too (it's still a video driver problem.) I'd recommend a temporary switch to 256 color mode to see if the problem clears up there as well.