with the game running it will run 100% of your CPU, assuming you have a duo-core, its 50% each one. To test it, stop the game, and see what it is, it will most likely be at the "pulse" around 1-2%
1 to 4% is pretty high when the computer is idle, with no applications loaded. Should be at 0%, and bouncing to 1% (occasionally 2%) at very regular intervals.