I don't like anti-virus softwares as they protect your phone from having viruses loaded onto it without you agreeing to load them. The entire system of S60 3rd edition is built around the idea that every application must state up front what parts of the phone it will access and will only be able to access those parts. Anti-virus software can't stop a virus you've agreed to load yourself so they're pretty much a con on S60 phones.
By now a few of you may have heard of the "curse of silence" virus going around Nokia smartphones. The virus isn't actually as bad as it sounds though so don't worry. Basically someone has to send you a multimedia or text message from an address with a certain e-mail configuration. Receiving this message will lock down the messages portion of your phone, or shut most portions of the phone down if it's an older model. It's not really a virus; more a bit of a security oversight in the system, which many of the anti-virus softwares out there are using to get more subscriptions from unsuspecting people.
So, how can you protect yourself from this "curse of silence" without shelling out a yearly subscription to an anti-virus software that you don't need? Well, you could try Nokia's own free SMS cleaner application, specifically designed by them to combat this problem. This is an important download that everyone should know about and, at only 42KB, it's one you can't afford to miss.