You will need to share a bit more about the hardware specs of your computer. Mostly are you using integrated graphics. If so that is usually the reason that Windows would reserve so much RAM for hardware use.
Also if you are using 32bit windows you will never see much more than 3GB available for use. But I have never looked at the resource manager on a 32bit machine so I don't know if that missing ~1GB shows up as hardware reserved or not.