On top of old-polacks wise words. sometimes apps/files etc etc just don't "release" themselves properly therefore the cpu will carry on holding onto those processes and whilst other new ones are being accessed it just overloads the system and occasionally its your poor old hard drive that pays the price

I must say it happened to me a couple of days ago and my widget was showing cpu usage fluctuating between 80 - 100% and my disk was being thrashed (reminded me of my old head master

)
I closed down the apps I had open - waited a few minutes - and after a reboot everything was hunky dory again.
Pleased to say this is a very rare occurrence.