Trojan Horse on my computer?

When my computer (XP or Vista) is idle for a few minutes, the read-write lamp begins to flash like a defrag or virus scan is running. When I open the task manager, a "system idle process" seems to be active. I have turned off scheduled defragmenting, windows defender and virus scans, and even in my travel laptop with no antivirus this "dreaming" occures. It must be a windows component as this happens even on a new computer, without any third party software. I'd rather not have my hard drive thrashing like this every day if it is not necessary. What the heck is this?

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