I had the same thing at the beginning of the week.
I'm fairly sure it was caused by a roll-over ad at the bottom of a Rootschat page, which said 'is your internet connection slow? click here to clean up your computer'. I didn't mean to rollover it and although I didn't click on it it started downloading anyway. Norton threw up a minimized message which I couldn't enlarge, and then I couldn't post anything at all as I was continuously returned to the 'Welcome Guest' page. I did eventually manage to post one reply to an RAF page but it appeared as a separate topic.
Nothing showed up on a scan so the only way I could return to normal was to start using my spare hard disk (my computer has two, both identical).
Michael J.