CPU Usage 80 To 100 Percent While Playing Games

When idle, my cpu usage is around 20%, which is allot more than it should be in my opinion. Whenever I start a game, Battlefield : Bad Company 2 which is the game I've been playing allot lately, my cpu usage doesnt come down from the 90's %. It just stays there and it often goes to 99% and stays there, once again. I've been running the game at a 20 fps average. When I could run games like far cry 2, cod mw , nfs shift and so on with settings on high with an average of 40 fps.