This is a list of some of the best free security apps, though some are intended for non-commercial use. While you can acheive good security with this collection, I would recommend a paid, protection suite from a reputible company like TrendMicro or Esset rather that the free programs if you can afford it. Check and see if your ISP provides anything and if so use it since you've paid for it already. Click here for a comparison of 5 top free antivirus. Find some open source solutions and security testing links too.
avast! Antivirus Home Edition - Quality antivirus protection though it does require registration after 60 days. 2000, XP, Vista, 7 (32/64 bit)
Avira AntiVir Personal - Very light-weight and particularly good at detecting new viruses. 2000, XP, Vista, 7 (32/64 bit)
BitDefender - Nice detection rate and now features on-demand scanning. Home Server, XP, Vista (32/64 bit)
ClamWin - Open Source protection for Windows, works on server editions too. 98, 2000, XP, 2003, Vista, 7
ClamXav - Open Source with same ClamAV engine but for Mac OS X. OS X
Grisoft AVG Anti-Virus - One of the most widely used though detection rates don't seem to be quite what they used to be. 2000, XP, Vista, 7 (32/64 bit)
Microsoft Security Essentials - Full-featured virus and malware protection from Microsoft. XP, Vista, 7 (32/64 bit)
Ad-Aware - One of the most popular free antispyware apps out there now with complete malware protection. 2000, 2003, XP, Vista, 7 (32/64 bit)
Spybot - Search & Destroy - Very simple to use but, less popular now because it leaves some known malware types behind. 2000, XP, Vista
Spyware Terminator - Has the option to integrate ClamAV during the install for real-time AV protection. Not listed as supported on 2003 but, seems to operate fine. The amount of notifications might drive you crazy though. XP, Vista, 7 (32/64 bit)
Windows Defender - Included free with Vista, honestly the detection rate is pretty weak in my opinion. XP, 2003
ClearOS - Network and gateway server designed for small business. Lots of intergrated services such as DNS, SMTP, and Webmail.
Untangle - Open source multi-functional firewall software. The 'Lite' or free package offers such features as web filtering, virus and spam blocking, and intrusion prevention. An awesome product but, not really suited to a novice user.
ZoneAlarm - One of the most popular, blocks uninitiated and unwanted traffic. XP, Vista, 7
HackerWhacker - Free tools that check for common vulnerabilities, ports blocked by your ISP, connection quality, and more.
ShieldsUP! - Provided by the Gibson Research Corporation, on-demmand scanning that tests your firewall for vulnerabilities. Very popular, be patient as you may have to wait for bandwidth to free up.
Unmask Parasites - Check your website for hidden links, iframes, malicious scripts, unauthorized redirects, and other sign of security issues.