Every time you carry out some actionon the PC the engine - CPU - has to work. For periods of fractions of a second or longer it actually works. The rest of the time is does little or no work.
What is normal for a computer.
If you do remodelling of financal data ll the time CPU usage will be high for significant portions, as would it when playing back a DVD movie.
When you are just typing emails, well it could just about go to sleep.
So don't worry: nothign is wrong. It's just the way a computer works.