>>Could it be the virus scans? Sometimes I think my home PC has locked up, only to see the Norton "file ... is awaiting a virus scan" pop-up on the task bar.
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>>Don't know why its CPU usage should sky-rocket though.
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>It could be, but symptomps are very close, so I have to investigate.

Try to turn one of them off. Maybe they're thinking the other guy is a virus and somehow get gridlocked. Reminds me of a situation where someone gives his house keys to two of his friends (but not telling either of them of the other) to feed the cat and water the flowers during his vacation. One of them is an early riser, and the other is a night bird, so he figured they'd come in at different times, if ever.

Accidentally they get into the house in the afternoon, not knowing of each other, and they notice there's someone else in the house - ah, must be a burglar! So they stalk each other, start a throwing things at each other, smash a lot of glassware and pottery, and then when it comes to hand-to-hand combat, they recognize each other, and, to celebrate a successful prevention of a burglary, empty the bar.