I would tend to think a virus might be causing this problem. I was working on a machine that got hit by the Klez.H worm. When i finally used Norton to get rid of the virus and tried opening any word or excel document, I got an error message similar to yours. What happens I believe is the worm overwrites with zeroes files with these extensions: txt, doc, xls, mpg, mpeg. Run a virus check on that pc and here is a link regarding the Klez.H worm if that is the case. <a href="http://securityresponse.symantec.com/avcenter/venc/data/[email protected]" target="_blank">Klez</a>