[intregus]

I don't on windows either.

[fortybillion]

Same here. I used Win 95 through XP without ever installing AntiVirus or AntiSpyware, and I never had malware.

[there]

...that you knew about.

[hxa7241]

I suppose if it is unnoticed, it is symbiosis -- like those hordes of bacteria (and viruses?) that live within and without all humans. (Assuming any remote harm would be noticed too.)