I have been using Antivirus software for a very long time and even though I kept it up to date it never catched a single virus, worm or Trojan. It did however slow down system startup and the scan of new files also used system resources. This let to my conclusion that I did not need a Antivirus software, because frankly, Antivirus software is basically preventing the user from doing something stupid.
What is your Security Concept ?
Many novice users use the software that comes preinstalled with their purchased computer and rely on these to be safe from the dangers that lurk on every corner in the Internet. Trojans, Malware, Spyware, Phishing and worse are threats that every user should know about and be able to deal with. It is pretty easy to detail my security concept, it is not the most secure on earth but even those are not secure at all. If you want security do not connect to the Internet at all, that is my advise.
