Some program is constantly running in the background. Open 'Task Manager' (usually 'Ctrl+Alt+Delete') and check for programs, Applications. Also, look at 'Start up' programs. Disable the programs that you don't need in startup (disable, not uninstall).
Disabling (or deleting) programs in startup will not uninstall the programs. They will still be available when you need them, but will not be loaded during startup. This will make the computer start up faster and will reduce the CPU load.