as you already have norton it is easier to just get the 2005 version. when you put the new disc in the drive it will say it has detected a older version and delete it and install the new version all in one go. you can get 2005 at pcworld.
Don't like to disagree with Fruit Bat who knows a lot more about these things than I do, but I've found a lot of friends who had viruses, etc. because they believed their AV would update "automatically". It depends on your usage of your pc. Maybe Avast is different in this respect (in which case humble grovellings) but it is not a good idea to assume that an AV will update automatically unless you are on line long enough & regularly enough for it to do that. Many of my friends use dial-up connections, go on line once in 3 days, maybe, for just long enough to get their emails, & then disconnect. As a result the update is not done unless they do it "manually".