if you're referring to the GUI then it's because the gui eats up a lot of resources. a single mouse move for example causes roughly 1000 lockings of the user handle table, then a bunch of locks for the gdi - not to mention the lower-level stuff in ntoskrnl. I'm working on this issue (especially the gui) in my rewrite branch.
Thomas
No, I'm talking about the CPU never being idle.
Casper