Here is a list of free antivirus software:

AVG Anti-Virus Free Edition
AVG Anti-Virus Free Edition is the most popular free solution available at no cost to home users and provides the high level of detection capability that millions of users around the world trust to protect their computers.
Website | Download

Avira AntiVir Personal Edition Classic
Free download of leading antivirus software package. Well suited for Windows XP, 2000. It detects over 250000 viruses.
Website | Download

avast! 4 Home Edition
avast! antivirus software is based on the ALWIL Software virus scanning technology since 1988.
Website | Download

ClamWin
ClamWin is a Free Antivirus for Microsoft Windows 98/Me/2000/XP and 2003
Website | Download