If you go to a site and a window pops up with no little x in the upper corner and it insists you press one of the buttons it's a virus. Even if the buttons are continue and cancel. Don't press the button. Click "CTRL + ALT + DEL" . Bring up the task manager and kill it from there. Same effect, only you know you're really in control. Any time a window doesn't do what you expect, kill it and go away.

If you get an e-mail or a webpage with links, check before you click. If the link says www.mastercard.com put your mouse over it. If the address in your browser shows up as www.hotbabesxxx.nl then don't click. It's easy to put a different link or even an executable on any text you want.

I'm running norton 360 and no virus for 3/4 years. It caught one in a zip game level last december. Blocked three intrusion attempts in the last month. I don't get viruses so much anymore. I get people trying to push viruses onto my machine through my port.
 
I'm not even that thrilled with Norton. It used to let you do all kinds of things. Now it's all automated. After I installed it said I was good to go. I found I still had to go through the panels and turn everything on. Vista blocked everything the install tried to do and the program couldn't tell me it was still idle. Anything is a lot better than nothing, especially when someone can just walk in over your internet connection.