I use the firewall built into my router. That's it.
I hear Microsoft's anti spyware is pretty good. I used to use Search and Destroy, but all it ever found were cookies that I didn't really care about.
My Rule of thumb: Don't download anything and you'll be fine.
I always download/visit obscure websites on my Mac using Safari. So i don't end up with any trojans on my PC. And there's no spyware on the Mac. Although I do use "Little Snitch" on my Mac which notifies me if any applications try to use the internet.
My mother, on the other hand, loves to download little applications (shareware) so Search and Destroy and Norton Antivirus are *required* but even then - she suffers. I say "Fix the USER, not the COMPUTER" . :-P
Joseph Elwell.