svchost.exe error

My son’s computer suddenly started giving him svchost.exe errors and now his search engines and several other services will not work. We initially thought the blaster worm might be the culprit, but after updating AV dat files and applying the fix patch, his computer is supposedly clean of the worm and there is no evidence that he ever had the worm. He says he has not added or uninstalled any programs prior to this error cropping up. What worries me is after restoring his system from an old and clean ghost image, he is still getting the error.