Security - Anti-Virus/Spyware
Bazooka Scanner - Program for scanning for malware - advanced.
Hijack This - Program for scanning for malware - advanced.
Kazaa Be-gone - program to remove Kazaa.

Microsoft Defender - Anti spyware program.
Spyware Blaster - Helps to prevent the installation of spyware.

Comodo AntiVirus + Firewall - Anti virus & firewall program.
SuperAntiSpyware - Underrated spyware program.
MalwareBytes - specialized malware removal program

Microsoft Security Essentials - Free antivirus scanner
Hiren's Boot CD - Bootable CD for system rescue.
Security - Spam Fighters
SpamFighter - email spam preventer.
SpamBayes for Outlook - Spam filter.
Zone Alarm - Firewall.

Sunbelt Personal Firewall - Firewall
Online Armor - Firewall
Security - Firewalls
Security - Miscellaneous

Site Advisor - program that advises when websites are safe, or not.
XP AntiSpy - utility that lets you disable authentication 'features' in WindowsXP - advanced.
Truecrypt - open source encryption program.
Microsoft Baseline Security Analyser
PCLoginNow - program that will reset the Administrator password.
Useful Free Websites

TrendMicro Online - Online virus scanner.
VirusTotal - Free online virus checker.

SimplyFreeware.ca
Disclaimer: I do not endorse any software that is listed here so you download and use them at you own risk.
Programs highlighted in RED are ones that I use on a regular basis.