You must find the process which is using all of your resources.
Ctrl+Alt+Del to bring up the task manager.
Under the processes tab, find the busy process.
You can end the process now. But if it is unfamiliar to you, search your computer for its name to discover the application to which it is associated.
The problem may be caused if you have media player and messenger updates or it may be Norton that is hogging your CPU usage.