Zoom To Large Size On Own

Dell Latitude 810D probably about 4 years old running XP Pro SP3. The following has happened a few times in last 18 months but reboot has always solved problem - today it continues even after reboot. No new software has been added recently; anti-virus program runs in background all the time and full sys scan every night. Dell tech support says it is a software issue. I am not getting any error messages, just screen magnification that has not been asked for by user.

When opening Firefox or MSWord initially the web page (or word document) looks normal at a reasonable text point size, but within a few seconds it progressively zooms larger to over flow the screen. In Firefox it looks like 1,600% in Word it seems to go to 500%. In either application using the menu or keyboard shortcut to restore screen to a more readable text size only retains that for a few seconds before it zooms and over flows the screen. Usually clicking on the scroll bar to move vertically is erratic and even though you click to go down on the page the screen seems to jump downward but then back up on its own.

As I said, rebooting typical would solve things but not today. Using the computer of a friend as my laptop is impossible to work with. Typing text in browser search, or Word, or email program because of the magnification becomes nearly impossible to track. I have not tried other apps, but the magnification is consistent except that with Internet Explorer things automatically zoom out making things at about 12% size.
Not sure this is an operating system issue but was not sure which forum to post this in. I am currently running a full system scan (Kaspersky Internet Security 2010). Desktop looks normal (icons and text labels look the same size that they always do; Windows Explorer also looks normal. Thanks for any assistance.