I'm amazed on how so many people think that antivirus software is a cureall its only as good as your last updates. Malicious users write malware daily to exploit and corrupt your systems. Turn on your auotmatic updates! YOu should also use a firewall, if your using wireless use WPA with strong paawords.
Spyware is also rampant on the Internet is your homepage changing automatically? There is also the threat of keyloggers which could be logging your bank account info, or even your Amazon account information! Use tools such as SpyBot Search and Destroy or Adaware.