HIgh CPU usage/slow pc

For awhile now i've been getting daily messages from Norton saying high cpu usage. its pretty much anything from games to browsers. My games are starting to white screen and have massive waves of lag. My browsers load quite slow, and the Pages "die" frequently. Anyone know what the problem may be or have any solutions to fix it? My Pc is very great i haven't had any problems with it. All this just started happening one day.