It's essentially impossible to calculate the draw of a computer, because you can't find out even what the maximum draw is for many of the hungry components, and measuring the actual usuage of each component can be tedious or impossible. Video card and mainboard makers don't release numbers for how much power they draw. Different CPU makers use different numbers for how much their processors use (some report maximum power dissipation at 100%, some report something like how much it "ought to use" under heavy load).
You can measure the actual power used the with the right type of meter. The simplest is the Power Angel probably. Plug it into the wall, plug the computer into it, get a reading. A clamp meter that just wraps around the power cord may be even simpler, but I don't know enough about electronics to know if it would actually give you the information needed.
For calculations, watts = volts times Amps. You could measure your actual voltage at the socket, or just assume 110 or 120 if you're measuring the total usage of the computer (or 220V in other countries).