After doing complete RegClean scan, Driver update, and Advanced System Optimizer scan with no change in symptoms, un-installed and installed Win Live Mail. Still no change in observed symptoms. Then performed system restore back to day before observed fault, noting that a "critical Windows update" occurred after that restore point, the evening before observing the fault on 28 Feb. (system set to auto install O/S system updates in accordance with default settings in Control Panel).
upon completing Restore ffunction, display returned to normal.
terminated auto install of Win Op Sys updates until allowed by computer user.