I was also experiencing several occurences of 'IE8 hung app' on my pc.
I didn't think I had any add-ons.
I noticed that very often, when I would get the hung app message, it would be when IE8 was opening a new tab or a new window.
I noticed that I had the google toolbar and I was using it, and somehow I had the ask.com toolbar installed, but not checked in the View > toolbars from the menu bar. Both toolbars had been installed in IE7 so I did not consider them to be 'add-ons', but apparently they are.
I went to Control Panel and I un-installed both toolbars and now IE8 is quick to execute and I have not encountered any new hung app messages.
hope that helps