Antivirus (or anti-virus) software is used to prevent, detect, and remove malware, including computer viruses, worms, and trojan horses.

Running an antivirus program is no longer an option but a requirement to keep your Windows system operational. However there are options available that allow you to stay free virus free that doesn't cost you any money ;-)

Additionally, for systems that seem to always be infected or systems that don't seem to be clean after updating and running your normal (installed anti-virus software) you can create a 'rescue CD'. This will be a CD that will boot your computer on a special operating system (one that will come on the rescue CD) to allow the software check for virus that hide once your normal system boots.

AVG

AVG - Rescue CD

Avast!

Avast - Rescue CD - Warring (NOT A FREE PROGRAM)

Mirosoft Security Essentials

Avira