Try going to C:\Documents and Settings\<your_username_here>\Application Data and deleting Mozilla folder, which will delete all Firefox's settings and start over. I suggest you backup bookmarks.html file from it first (it has all your bookmarks saved in there), after deleting the folder and running Firefox it will recreate it writing all the files again, go back in there and drop your old bookmarks.html in and you should have all fresh settings and your original bookmarks.
I also suggest you switch to AntiVir ( http://www.free-av.com/ ) it is faster, free, and has better capture rate.
I also suggest you switch to AntiVir ( http://www.free-av.com/ ) it is faster, free, and has better capture rate.