Windows Task Manager shows that the CPU usage keeps changing from 2% to 100% cycling every few seconds, even when the computer is first turned on, with nothing running. It creates stuttering in some streaming programs, and at times when it shows 100% usage hesitates until the percentage goes down.

How does it behave if you boot to Safe Mode with Networking? If better then something is starting at boot that is causing the problem.

Advanced startup options - XP
Advanced startup options - Vista
Advanced startup options - Windows 7

Use msconfig to determine what is causing the problem

These are good tutorials on using msconfig in XP, Vista or Windows 7:
How to use msconfig in Windows XP
How to use msconfig in Windows Vista
How to use msconfig in Windows 7

Click on Start then Run, type msconfig and press Enter.
Click on the Startup tab, record what is currently starting then click the Disable All button.
Reboot and see if it runs better.
If yes then use msconfig to enable several items at a time till you find the culprit.

If no, start msconfig and click on the Services tab.
Check the Hide All Microsoft Services box, record what is currently starting then click the Disable All button.
Again, do a regular boot, see if it runs normal.
If yes then use msconfig to enable services till you find the culprit.

Once you've found the culprit, uninstall it or find out how to eliminate it from your system. Simply disabling it in msconfig is a temporary fix at best.
Enable everything else you disabled.

Using msconfig, I disabled all startup files and enabled them 4 at a time but to my surprise I did not find the culprit. However, after doing this, the trouble seemed to stop. The CPU now cycles from 2% to 28%, which is probably normal. I will see if it acts up again while surfing which will mean a new problem. Thank you so much for your quick response. Neil

Post back if the problem reappears.