avast! antivirus. Free.. works.. Doesn't clog the crap out of your pc like Norton (unless Norton has lightened its load in the last few years, that is )..
2 add ons for Firefox that help with ads are Ad Blocker plus and Webmail Ad Blocker. Download and run those. Them, running Avast always, and running Adaware/Spybot once a week should keep you running clean and fast.