The video driver.
ATI Radeon X1200 series.
"The display manufacturer hasn't made your display compatible with Windows 10."
How could they make it compatible when it's not out yet?
Ran into the same problem with older Nvidia chipsets at my office. If you're feeling brave... here's a way around it: Install Win10 using the media creation tool. This will bypass the compatibility check. Once you boot into Win10 for the first time, it will be using the default display driver at 1024x768 resolution. Then, install the last Win8.1 driver available for your video chipset. It should work and get you back to a reasonable resolution..
So upgrading to 10 then reinstalling the vista driver worked for you?