Thanks Steve, I updated in the middle of the week after, I'll check again.
I've never had a virus on my machines, but it did pick one up (windows variety) on a memory stick. I didn't use anything for years, but I share files frequently and don't want to pass stuff on to my friends. I used Clamxav for a while but it did start to slow things down.
I think trojans and viruses will become more prevalent on Mac in the next few years, though because of the UNIX underpinnings I don't think they'll be as problematic as on Windows.