Number one method of avoiding computer viruses and spyware is to not fall for something stupid from scammers and hackers. I'd assume that if someone put out a virus in a mass email that simply read "Click here to install latest virus" there would be millions of people clicking on it.
At my work viruses originate from the same few users while most others have no issues year after year. Can't get a virus unless you click on something you shouldn't. Best defense starts with the user educating themselves on the do's and dont's.
