Search around for the x64 drivers or go here: http://www.eightforums.com/hardware-drivers/16245-old-acer-benq-scanner-drivers-x84-x64.html
Ignore the heavy duty INF instructions and just do this: because they're unsigned you'll have to 'advanced restart' then select #7: run without checking device signatures; load the drivers--it worked!!
This was a new Dell 2013 running Windows 8 64 bits! I downloaded GIMP 2 (freeware) to actually run the scanner instead of the old Mirascan software: File > Create > Scanner/Camera > select flatbed and it works. So happy that in the year 2013 my old flatbed STILL WORKS!!