I've had this happen, and one other twist. When I scrolled a document up and down, the text would, for lack of a better word, "dissolve". Literally, pixels would drop out of the text and it would be unreadable. After several things tried, including a reformat and a reinstall of everything on the machine, I was able to trace the problem to a single corrupted font (called "Brady Bunch") which Windows read as Arial, and replaced it in every application that used Arial as a default font. It made using Reader and Firefox impossible until I uninstalled it.
There is a font that Windows is substituting for the default Tahoma or Arial font on this system. I'd bet dollars to doughnuts on it.