CPU usage suddenly jumps to 100% with nothing else running! I know this can't be good. HELP!?

don't know squat about computers, only that my boys needed one for school. Have a Dell Dimension 3000, Windows XP Pent 4 and it's black. Otherwise, I have no idea what makes the thing tick. Only that everything started going wrong on the day we hooked up on the internet via SBC Yahoo! AT&T internet service. One problem after another since that day last month. Have spent more hours on phone with tect support then online. They send new software, tell me all is well with that problem only to have different problem pop up minutes later. Have lost all respect for their support teams. Trying to pay my way with Yahoo by answering questions. Over 50 questions past two weeks, many top answers, so in my field, I am no dummy. Computers, that's another world. Now I'm turning to you for help with my problem. This CPU thing is totally beyond me. Thanks! rob-hansen@sbcglobal.net A.K.A. "garden dok" at Answers.