A few days ago after exiting a windows popup I was left with a black line on the screen corresponding with one of the borders of the popup. Usually I only have to reboot for this to disappear, and whilst I have done so several times the line returns on my screen. This happens after a few minutes or even after an hour or so, but does not correspond to any subsequent popup. The line is always in the same place, about a third of the width of the screen on the left hand side. OS is XP Pro.