While most AntiVirus-Software is based on a list of known threats, ThreatFire tries to analyze the behaviour of programs to detect whether they are genuine or malicious. Together with Avast, it makes a perfect match!
Steps to install ThreatFire
1. Download the software from http://www.threatfire.com/download/ 
On that page, click on the "Get Free" button 
to start the download. You will be asked to "Run" or "Save" the file, select "Save". Click on "Browse folders" and save the software possibly to the Public Download folder of your computer (on Vista) or on "My Documents", "My Received Files" in Windows XP.
2. When the download is finished, click on "Run" to launch the Installation program.
Windows will ask you for confirmation to run this program, accept it.
3. Accept all the default suggestions during Setup by clicking on Next or OK.
