High CPU Usage

As I am treading on unknown ground I will just open my mouth let it roll. My computer has been very slow recently and I happened to open Task Manager and the total CPU usage at the bottom was 100%. It died down to 4 or 5% as it set there. I closed the Task Manager and reopened it again immediately and the bottom total was again 100%, slowing down to 3%. I then opened Performance and noticed a zig zag graph. I opened Processess and my Spy Sweep, which is minimized, was constantly going up to the 80s and 90s and down to a low number. I maximized it and closed it and the Performance graph was almost a steady 3. I then began to operate the computer and it was back up to speed. I opened Internet Explorer (I had temporarily un-installed Firefox to eliminate it as a new installation causing the problem, although it was showing in the high 20s) and IE quickly opened the Home page as normal. But, I then went back to Task Manager with IE running and there was another zig zag graph. I opened Proccesses and IE was the problem with the total going up in the 80's. Is this normal? Or do I have a problem with IE? I want to be sure the computer is in good shape before I re-install Firefox. I ran a Virus check and there were no viruses discovered. I will appreciate your comments and thank you. Woody