Bashed my head against the wall for a while with this... seems the latest Windows 10 updates are messing up the drivers for older ATI cards (Radeon HD4000 series and older). Windows 10 now seems to prefer the default "Generic VGA" driver over the legacy ATI drivers. Which disables 3D accelleration. Which royally messes up any 3D gaming and prevents your monitor from being autodetected properly (so the resolution drops to 1024x786)
To fix it: Go into the device manager. Find your videocard, select properties, update the driver. Tell Windows you'll manually search, then manually select. Then select the legacy driver from 2015 rather then the generic VGA bullcrap from november this year which doesn't work.
*one annoyed support tech that is very close to disabling W10 automatic updates alltogether if this keeps up*