Hardware devices can be easily managed in Windows using the Device Manager. I am interested in completely powering off a PCI device (my graphics card, for instance). I am aware that, in Device Manager, a device can be 'Disabled', for which this documentation exists
Disable a device
When you disable a device, the physical device stays connected to your computer, but the device driver is disabled. The drivers are available again when you enable the device. It can be useful to disable devices if you want to have more than one hardware configuration for your computer, or if you have a portable computer that you use at a docking station.
Does "the physical device stays connected to your computer" mean that the device is still powered on? Can this even be done through Windows, or is power management of PCI devices BIOS-related?