Could you please post the spec of your PC so we can see what we are talking about here.
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You can also try a Basic Windows Theme, This will disable easily desktop compesisition that can lag games.
Type Power Options into Start menu. Choose a High performance mode on the extra plans. This will disable 1 of 2 motherboard services that decrease
power to save electricty
However from the sounds of it you need to Hardware upgrade before you think about windows 8. Windows 8 will give you a 10-15% increase in BF4 performance tho. If you are on a bad rig anyway the difference will be minor.