My cou randomly maxes out?

i look for this question all ready and havent gotten a answer but my computer cpu usage will randomly max out at 100% and be stuck like that till i restart it or wait forever for some reason im not running any thing high performance no games since it started doing that but ive tried reformatting the comp and still dose this. dose any one know wahts wroung. these are my computer stats: intel celeron processor 1.6 ghz, 1.49 gb ram, acer aspire 3620, phoenixBIOS 4.0 release 6.1, display intel915gm/gms 910 express chipset, windows xp home 5.1 build 2600.