On Windows XP SP3, in Task Manager I assume that the "System Idle Process" is not counted as part of the overal "CPU Usage", therefore at any time the sum of both should be 100%, which seems to be true most of the time.
However sometimes, the CPU Usage may be at 50% or more while the idle process is still using 90% or more! For example when I start a program on my dual-core, I see one CPU going to 100% and the other staying fairly low, so the overall usage is around 60%, but the idle process is still above 90%.
What could be the reason for this discrepancy, and what is my CPU really doing at that time?
Note: I've checked the "Show processes from all users" box in the Processes tab.