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?
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.