Is your OS up to date? Has it got service pack 2 installed?

To go back to IE6 you could a quick restore, if you don't trust that then search Google or Microsoft for it.

It may even have something to do with spyware or a virus.

A good pointer for viruses is check your processes. Google any exe.s you're unsure about. I know you extensive cleaning but there is a small chance that it could be undetected.

The final thing I can think of is your computer maybe just plain awful.