Security Essentials from Microsoft is a great anti-virus and anti-most-everything-else program. It detects and removes most Windows viruses, trojan horse programs, root kit viruses, and most other malware.
NOTE: Malware is an abbreviation for "malevolent software." Malware has many varieties, but all have the same purpose: to do damage to your computer and then (usually) ask you for money to fix it. This is a high tech version of kidnapping. It is illegal, but most malware creators never get caught.
I wrote about the free Security Essentials from Microsoft in the October 1, 2009 newsletter. It is free and does a good job. However, a new malevolent product is floating around with a similar name: Security Essentials 2010. Note the "2010" tacked onto the end of the name. The product from Microsoft has no date; it is simply called Security Essentials. The bad program written by unknown scam artists adds the date on the end: Security Essentials 2010.