[ubuntu] 50% CPU usage, but no processes using CPU

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http://ubuntuforums.org – My netbook is running slow, and System Monitor shows ~45% usage on each of two CPUs. However, the "Processes" tab of System Monitor shows all processes at 0% CPU usages. Any ideas for tools to help me investigate this? (Using UbuntuEEE on Asus EEE 900A) (Hardware)