iMac Running insanely slow.

My iMac is running incredibly slow as of late. It is about 2 years old. It has always run very well until recently. About every 30 seconds or so, something is running and taking up all the CPU. I have rebooted, and loaded the latest Norton and made sure that the OS is up to date. It doesn't change. What happens is the curser stops and spins every 30 seconds or so indicating the computer is busy. And indeed the computer freezed during this for about 20 seconds. I checked the CPU usage and it doesn't show anything. Any ideas?

I am not sure what to do next.

Thanks,

R Hedrick