When is the system is turned on, there are various process running on the background which helps the system to function properly. Apart from these processes when other things are done like copying, moving a file, opening a program or playing a game, it literally brings in new process to existing ones.
Cpu usage increases. How the process is handled and how smoothly depends on the type of Processor you have, either a dual core or a quad core.
Here is how to check, your CPU Usage :
No software is required, its already there in the windows itself.
Right click on the taskbar > start task manager > Performance
You can observe the graphs of your CPU performance. Basing on that you can know the limits of your CPU's performance ideally.
Do post your comments and critiques !
Also check What are the advantages of having Mutli Core Processors(cpus) ?