What video card? Some onboard video grabs RAM. My guess is something along those lines... to actually figure it out we probably need a full system breakdown, it will most likely end up being some mother board component using the RAM.
32bit operating systems only use a maximum of 3Gb as they can not address more than that. only a 64Bit OS is capable of using more than 3Gb RAM. However this does not account for the missing 1.05Gb! - Just noticed you wrote Windows 7 64Bit!