If you reseated the CPU and tried different RAM already it could be you have a defective motherboard. I read some other threads where that was the case.
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Hit ctrl-shift-esc to quick open tasks/performance. Are apps using a total of 8 gb? What are you running?
What other hardware do you have installed in your system?
Can you take a screenshot of the Memory tab of Resource Manager?
It looks like the BIOS is reserving the memory for something. Reserving memory is pretty normal, but 8 GB isn't. Have you tried resetting your bios to default settings.
I had this issue once on a PC a few years ago. I ended up having to slightly over volt the RAM in the bios. After doing that I no longer had any issues. By slightly I mean I upped it by the smallest increment once or twice.
I know this might not be helpful.. but umm.. have you tried nit using windows?
Since the PC shows the full 16gb it must be registering it. So something inside windows must be occupying the rest of the space.
I've seen windows alone go up to 4+ GB so the ~8GB your missing here aren't unrealistic depending on what programs are running in the background.