From my personal experience, it is really a good idea to just buy one, if you don't visit bad websites. Because installing too many of these softwares seems to slow down the computer speed and eat up spaces and memory.
But here is the list of best free softwares out there:
http://www.download.com/ZoneAlarm/3000-10435_4-10039884.html?tag=txt
http://www.download.com/SpywareBlaster/3000-8022_4-10196637.html?tag=txt
http://www.download.com/Microsoft-Windows-Defender/3000-8022_4-10353597.html?tag=txt
http://www.download.com/AOL-Active-Security-Monitor/3000-2094_4-10545561.html?tag=txt
http://www.download.com/Avast-Home-Edition/3000-2239_4-10019223.html?tag=txt
http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10320142.html?tag=txt
http://www.download.com/HijackThis/3000-8022_4-10227353.html?tag=txt
http://www.download.com/CCleaner/3000-2144_4-10315544.html?tag=txt
http://www.download.com/SpyCatcher-Express-2006/3000-8022_4-10436704.html?tag=txt
http://www.download.com/TrueCrypt/3000-2092_4-10527243.html?tag=txt
Following are the ones I found thru about.com:
AVG http://www.grisoft.com/us/us_dwnl_free.php
Here, you can get your free copy of the AVG 7.0 Anti-Virus System - AVG 7.0 Free Edition and you will be able to use it without any limitations for life of the product.
Avast Home Edition http://www.avast.com/eng/avast_4_home.html
avast! 4 Home Edition is a free antivirus software for home noncommercial use. It scans for viruses, worms and Trojans on disk, CDs, in E-mail, IM and P2P . Incremental updates of virus database (twice a week) are small, fast and reliable.
AntiVir Personal Edition http://www.free-av.com/
The AntiVir Personal Edition offers the effective protection against computer viruses for the individual and private use on a single PC-workstation.
Clam Antivirus http://www.clamwin.com/
ClamWin is a Free Antivirus for Microsoft Windows NT/98/Me/2000/XP/2003. It provides a graphical user interface to the Clam AntiVirus scanning engine.
ScripTrap http://keir.net/scriptrap.html
ScripTrap traps scripts when they attempt to run on your computer and provides the option of blocking them or letting them continue to run. You can also check the intercepted script with your anti-virus program before you decide to run it or not.
Trend Micro Online Scan http://housecall.trendmicro.com/
Give your PC a FREE check-up! HouseCall is a demonstration of the power of Web-based technologies that Trend Micro is developing to make deployment and management of virus protection in corporate settings fast and easy.
McAfee Stinger Virus Removal Tool http://vil.nai.com/vil/stinger/
Stinger is a stand-alone utility used to detect and remove specific viruses. It is not a substitute for full anti-virus protection, but rather a tool to assist administrators and users when dealing with an infected system.
Symantec Virus Removal Tools http://securityresponse.symantec.com/avcenter/tools.list.html
Symantec Security Response has developed tools to automatically conduct what would often amount to extensive and tedious manual removal tasks. Check this link for a list of virus removal tools.
BitDefender Virus Removal Tools http://www.bitdefender.com/html/free_tools.php
SOFTWIN provides you with a powerful set of Virus Cleaning Tools, designed to detect and remove viruses that infected your system. These applications are also valuable because of their size, making them easily downloadable even with a slow Internet connection. Check this link for a list of virus removal tools.