Yesterday I bought a Toshiba Netbook and I purchased the Pro Version Of Avast. There are no other AV programs on here. I even uninstalled the Norton that install that they want you to try. This is a brand new computer with NOTHING installed on it other than FF. After I installed Avast 5.0 my computer is crawling. Its even making the letters I type in this message show up 10 seconds after I type them.
When you uninstalled Norton did you also use their Removal Tool? If not, Google “Norton Removal Tool” and there will be a link to Symantec for a download. This removes things a normal uninstall fails to remove and is known to cause problems. Worth the try!