Well, at least you have eliminated IE as being the culprit. It definitely sounds like it could be spyware, but you've done everything, and Dack would have spotted anything iffy in your Hijack This log. It sounds like your system is struggling to keep up for some reason.
Have you recently loaded any software, or anything? How are your resources generally? How much disc space do you have left? Not that you download much, of course. :whistle: Is there anything in Task Manager which shows as using rersources heavily?