Problem with desktop background

System boots normally, but last three boot screens show the normal background, then a blank desktop except for the desktop icons, then a combo screen with the icons on top of the background. Except this last screen also has a white, approx. 2", square block covering part of the background. No matter what background I use I still get the square. This is not my machine (it's the kids) so I have no history except that this just occurred. I tried the restore command to go back to 12/1, but it said there were no changes and the kids insist the problem was not there last week. I looked in msconfig/startup and it lists a slew of items, but many are acronyms and I did not know what they meant.