I have a nack for keeping my PCs alive. I actually have one that is ten years old and has never had a virus on it. Of course, she is retired, and I'm on my new laptop now.
Common sense is the best way to avoid a virus. Do some research about a program that you have never heard of, or seems a bit shady. Look for reviews of the app on the page you are downloading from, and look for reviews on the website that is hosting the file you are downloading. If there are a bunch of people posting, "This thing has a virus! I had to re-instal windows! ZOMG!", you best not download it. Heck, even if there is only one review that says that, you best not download it.
Also, I used Norton in the past, and now use McAfee. I must say it's like going from the Wright flyer to an F/A 22 Raptor.
I won't suggest going into RegEdit or messing with the registry unless you know EXACTLY what you are doing. I hardly ever mess with my Registry, mainly because I don't know it well enough, and there has never really been a need to do so.
I've been using computers for eleven years and have never caused a problem with the Family Computer, or my own. Follow the advice given by the guys above me, and my own, and you should be safe. Heck, McAfee just finsihed scanning my PC. 128185 items scanned, no problems detected.
P.S. Window's firewall is your friend! (Unless you have a Mac.)