Open up task manager, go to processes, and sort by CPU usage (percentage). Try to find out which program it is by looking at the name and description, then check if you actually need it or if it's something using CPU that you don't need.
If you don't know what it is, then you probably shouldn't touch it. The "System Idle ..." takes up all the remaining CPU, don't try to stop that.
Two random images from the internet that show what I mean: