you probably have a few zombies (sub processes that are still running but their parents were killed off, they are lost and not connected to any primary process) these are running in the background. Memory leaks and CPU usage are two different things. Memory leaks can kill performance too. If you have free memory and a lot of processes running in background just do a restart or logoff and re-login. That should kill any zombies. They may re-arrear if you load multiple IE's and are doing a lot different things and not cleanly exiting out of them.
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