Thank you very much Antonius:
You are correct in your previous post that grahics cards and drivers are the highly suspects for this error. When I called MIcrosoft support, the tech also told me that my video card could be the cause of the problem. The graphics card was one of the first items I checked when I first noticed the problem - its status was reported as good and the last update was reported as yesterday; so I didn't think the graphics card was causing the problem. But after I read your post and ruled out that my license couldn't be the problem, I went to the Nvidia site and downloaded a new driver (320.00 quadro-tesla-grid-- --) to replace the old driver (268.02-.quadro---) that came with the computer. After installation and restarting, the problem went away and I can now run both WF5 and the Autodesk Revit that wouldn't run. Thank you so much for taking the time to help.