Here is an article from US Cyber Security. The title is "Cyber Security Tip ST06-002: Debunking Some Common Myths". I have listed the myths they cover below. Go to their site for the truths.

Link to article: http://www.us-cert.gov/cas/tips/ST06-002.html

* Myth: Anti-virus software and firewalls are 100% effective.
* Myth: Once software is installed on your computer, you do not have to
worry about it anymore.
* Myth: There is nothing important on your machine, so you do not need to
protect it.
* Myth: Attackers only target people with money.
* Myth: When computers slow down, it means that they are old and should be
replaced.