Windows 8.1 Display Driver for Extensa 4420

I've got an Extensa 4420 that I've upgraded to Windows 8.1 in a dual-boot configuration w/ Windows 7.  In the process, I've lost 25% of my screen pixels when running Win8.1.  When I run Win7, the system gives me a 1280 x 800 pixel screen, and detects the Radeon Xpress 1250 GPU.  When I run Win8.1, the system gives me a 1024 x 768 pixel screen, and insists on running "Microsoft Basic Display Adapter."  The screen is completely filled in, but is distorted and "clunky" looking.

So I guess the question is, is there a driver somewhere for the Radeon Xpress 1250 GPU that works for Windows 8.1, and, if so, how do I force Windows 8.1 to use that driver?  FWIW, I've tried to update the driver using Device Manager, but the newest driver I can find is for Win7 (on acer.com), and Win8.1 doesn't like it.  AMD/ATI doesn't seem to have any updated drivers for that specific GPU.

I suspect I should just give up on Win8 for this laptop, but it could be a good place for "adapting" to Win8, if I could make the display look better.

TIA for any insights.

John B