Keeping a computer virus free is pretty easy, but there is no way most people will do it. A virus cannot magically appear on your computer. Most of the time it comes from the Internet through some mechanism.
1) I have a hardened firewall at home. It only allows port propagation on the ports I choose. All others are blocked.
2) I disable many (most) features (activexx, java, javascript...) of my browser. Basically if it aint pure HTML, it aint getting looked at.
3) My email gets scanned by my firewall/router (Linux box, by the way) and stripped of any attachments or HTML or any other scripting language. If it aint ASCII, it aint getting to my computer.
4) If a program requires ActiveX or .NET, it never gets installed. I have a test system (not on my network) for checking beforehand.
There are more details, but I figure the gist of it will cause most people to shy away from doing it. Afterall, you will not be able to view 'youtube' or any other videos on WEB pages after you are done. But, those can be a source of viruses, so I'd rather not deal with it.
The tradeoff for me is I get better performance, higher reliability (the more carp you ad the worse things get), and I really am not giving up anything worthwhile.