Your best protection is <strike>common sense</strike> Linux.

I don't believe that antivirus or anti-spyware software can truly provide protection, and as such, I haven't used any in the last five or so years, and haven't had one virus (that caused noticeable activity - without the software, you don't know about infections ). If you're careful about what you download and install, you're mostly safe. Realistically, nobody cares about "hacking your mainframes", as software firewalls would have you believe - the only point of concern for me (if I used Windows) would be questionable applets or ActiveX controls running on a webserver, that could install bad stuff when you're not looking. But you have browser-specific settings to prevent that if you think there's enough concern.