You aren't running any sort of proxy? I run the proxomitron, and it has the same symptoms at times. As long as you've installed a fresh browser, I doubt that would be your problem. Am I right in reading this that the sites themselves respond, but only certain pages are screwed up?

Can you try going to a page that doesn't work right and saving the html source and then going to the same page from a working machine and seeing if the source matches?

Matthew