After installing updates in Windows 7, I found that my primary display flashed on and off rapidly and I was unable to activate any menus on the screen. My extended display, powered by a StarTech USB to VGA Adapter, remained off even though the drivers were properly installed and everything had been working prior to the updates. Unplugging the StarTech adapter restored the primary display to normal but left me without a second monitor.
The recent update had included dozens of individual security updates so I researched the issue online and found the symptoms linked to the following Windows update:
This is a graphics performance update that has had a number of reports of compatibility problems and other issues.
My solution was to simply uninstall the update. This required a restart of the system, after which both monitors worked fine. The KB2670838 update is a prerequisite for Internet Explorer 10 / 11 and removing it will cause these versions of IE to malfunction or be removed from the system. Re-installing IE 10/11 will also reinstall KB2670838. Removing it did not affect my installation of Google Chrome, however.