Think of it like this: your computer is like your body. It has to be cleansed of contaminants introduced to it, or they must be blocked completely.
Your body does this automatically. Your computer? Not so much.
Scan every foreign file before running it. If you get a virus warning from an antivirus program when you run a program, abort the program immediately, delete the viruses detected, run a full, yes, full scan on your computer, and most importantly, delete the file and clear your recycle bin.
Run a full scan once every week, update whenever updates are available.
This may seem excessive, but I speak from experience. The two hours a week and extra right click on downloads could keep you from loosing your computer for a month.
Ex 1: I downloaded a .zip just yesterday (probably with a hidden .exe, didn't check), and MSE spotted and alerted me to the virus threat immediately. I was about to run a scan anyway, but it noticed the threat before I ran the scan.
Ex 2: I got a virus on my desktop at one point that claimed every .exe was infected. It was a major pain in the ass hunting it down.
Ex 3: A minor virus gets in. Says you need this "official" Windows update. It's actually a major virus. Your computer is now completely infested.
