Viruses are bad. Everyone knows it. How bad, well... It seems that computer viruses are truely an international epidemic. I think the largest indicator of their severity is the diversity of countries which produce some very high quality virus software!
My current recommendation is for Avast!. They support both Macintosh (PowerPC and Intel) as well as Windows, and now even Linux/Unix platforms. The virus software is fast, it updates reliably, it is reasonably priced.