While I'm neither constantly running pirated software I've downloaded nor ever running IE, Outlook, or Office (using Firefox, Thunderbird, and OpenOffice.org instead), I've had zero problems with just running AVG's free anti-virus and having a hardware firewall and every once in a while run AdAware and SpyBot. I never actually get any viruses or spyware, so I could probably do without AVG and AdAware/SpyBot, but gives me a small sense of security.