I've been having the same issue recently and have tried MS fixes as well as some mentioned here... AVI, virus scan, spysweeper, etc.

I believe I finally have it licked though and it was something I wouldn't have guessed. I ran Ad-Aware SE and went ahead and deleted the non-threatning MRU lists. Voila, no more 100% lock ups.

Hope this helps someone out there since I know this drove me nuts.