I use and would recommend the Intego SecurityBarrier (VirusBarrier and NetBarrier) software for the Mac (
www.intego.com). It is simple to install and configure; and, most importantly, is completely unobtrusive. The latest version X5 is Leopard compatible. There are 30-day trial versions available for each of their products.
The Intego solution is not free, you pay for the software which includes the first 12 months subscription; but the subsequent renewal is reasonably priced and there are good discounts for taking out longer subscriptions.