OK, there's a BIOS setting that will limit memory in the manner in which you are experiencing, it was a compatibility flag and I remember running into it a few times back in the day. Do you happen to know your motherboard model # off-hand?
Also, I'll echo what many others have said, get Windows 7 off there, upgrade to Windows 10 or do a fresh install of Windows 10 (there's a guide I created on here, pinned post) and activate it with your 7 key. You are at significant risk running an OS that no longer receives security updates.