when i encountered this kind of problems, the CPU utilization was usually less than 5% and memory, I/O activities were all normal
it seems that explorer.exe is just not redrawing the desktop, including the taskbar
typical case is that when the user clicks a button on the quick launch bar, that particular button caves in and remains in that status for 30 seconds.
ending and restarting explorer.exe will solve the problem once, but same thing occurs again very soon.