CPU usage to the max

Hi,
I’m having dificulty with my CPU usage (I have Win XP Home Edition), when I open the Task Manager and select the Performance tab, the CPU usage is up and down from 24% to 100%, and i only have a couple programs open!!! I’ve scanned my computer for viruses and nothing, all clear!

What should i check next?
Can someone help me with this???!!!

Thank you!
LP
Cayman Islands