How did you know? I'm presuming you used an online scan or something like that... otherwise, you'll be amazed at what lurks unnoticed.

I left a new install of XP unvirused, unpatched and unfirewalled for about 60 minutes and 8 viruses/trojans found *their own* way in (presumably through security exploits... if you run a firewall or are behind a router then just being careful is enough to avoid viruses most of the time)

As for Mac, no worries whatsoever from what I've seen so far.