A few days ago a sand clock appeared near my mouse curser when my comp was in idle. I checked the performance in the task manager and I saw that the CPU usage jumps from 7% to 100% every 6 seconds. Strangly enough, when I connect to the internet, the situation returns to normal and the CPU usage is around 6%. When I disconnect, the CPU usage continues its jumping around. No viruses were found with my fully up-to-date Norton Anti-Virus. Any ideas? OS XP Pro, 1.7 Ghz, 512 MB Ram.
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Could be some Spyware/adware. Try downloading Ad-aware and Spybot and run those. They're not technically Viruses so wouldn't get picked up by Norton.
You might also try spy sweeper from web root. I had a similar problem and norton, adaware and spybot could not find it. I downloaded a trial of Spy Sweeper and it did the trick. Here is a link for a free trial.