If a customer does not have a ruler on the screen, no typing appears, there is a thick line on the left hand screen, no margins, and no cursor but the mouse moves and old files seem "intact", try the following:
1) Change the video driver to VGA
2) See if they are using HM520 printer driver as default. If so, change the driver default. As long as the HP520 is not the default printer, this problem won't occur.