Drivers

While Windows will probably install drivers for your GPU, these drivers are usually lacking the necessary components to do calculations on that hardware. This is done to make the drivers be available for a wide range of hardware. It's also that a lot of the components are open source, a thing that Microsoft doesn't like, and most of these components are direct competitors with things that Microsoft makes:
- OpenGL -> DirectX
- CUDA/OpenCL -> DirectComputing

So it is better to use the drivers from the GPU's manufacturer, being AMD (or ATI), Intel or Nvidia:
- AMD drivers
- Intel drivers
- Nvidia drivers

Wherever, try to make a clean installation of the drivers. This may be an option in the driver installer (mostly in the case of Nvidia drivers, and it's hidden away in a non-obvious place so read all the options carefully), but it may also require you to uninstall the previous drivers via Device Manager or Uninstall a program.