Check the specs of the motherboard. It is probably limited to 32-bit. This means that you can install only a 32-bit OS, and 32-bit systems can address only 4 GBs of RAM, of which a small amount is taken up by the system.
This is why you see only 3.68GBs of RAM even though you have installed 8 GBs on the motherboard.
If your motherboard is 64-bit capable, then you can do a complete (Custom) install of the OS. This means that you will need to re-install all your other programs too. So, make a backup before you do this.