"…antivirus programs have been getting less effective as new viruses are more frequent and existing viruses mutate faster. Yes, antivirus companies are forever playing catch-up, trying to create signatures for new viruses. Yes, signature-based antivirus software won’t protect you when a virus is new, before the signature is added to the detection program. Antivirus is by no means a panacea.
On the other hand, an antivirus program with up-to-date signatures will protect you from a lot of threats. It’ll protect you against viruses, against spyware, against Trojans — against all sorts of malware. It’ll run in the background, automatically, and you won’t notice any performance degradation at all. And — here’s the best part — it can be free. AVG won’t cost you a penny. To me, this is an easy trade-off, certainly for the average computer user who clicks on attachments he probably shouldn’t click on, downloads things he probably shouldn’t download, and doesn’t understand the finer workings of Windows Personal Firewall." – Content courtesy of Schneier on Security: Is Antivirus Dead?