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I'm using WinXP, and I notice that in the Task Manager, on the Performance tab my CPU usage is 100%. But When I go to the Processes tab, and sort by CPU System Idle Process is taking up 90%. I've double checked that things like AVG anti-virus isn't actively scanning, and that jkdefrag is not defragging. But I know something is taking CPU cycles, because the machine is very slow. How do I determine which process is hammering the CPU if it doesn't show in the Process List?

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up vote 12 down vote accepted

Process Explorer is a much better tool for inspecting processes than the built-in Task Manager. While it might not answer your question completely, it may give you more insight into the problem.

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+1 Great tool. This allowed me to see system level services thrashing the hard drive. SQL Server in particular. – nathan Jul 30 '09 at 13:05

Make sure that the "Show processes from all users" checkbox is checked at the bottom of the process list in Task Manager, so that all processes are show.

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I was having a similar problem a couple of days ago and it turned out the machine was infected with a Rootkit. Since removal things seemed to have calmed down.

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