Uninstalling an antivirus or any other program should always be done through add/remove programs or by using a free third party uninstaller like Revo Uninstaller. Only use the below methods if the add/remove programs method fails.
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• AVAST! Removal Tool - avast! uninstall utility

• AVG Free Antivirus:
  • RECOMMENDED BASIC AVG UNINSTALL INSTRUCTIONS
  • Anti-virus and Security Removal Tools
    

• Avira' Antivir Registry Cleaner Tool- Avira AntiVir Removal Tool

• BitDefender Antivirus Removal Tool:
  
  • Download the BitDefender Uninstall Tool.
    First right-click on the red BitDefender icon near the system tray choose on Exit.
    Wait a few seconds. Afterwards double-click on the BitDefender uninstall tool.
    You will need to reboot your computer for the changes to take effect.

• CA Internet Security - CA Internet Security Suite 2008 Uninstallation Instructions

• Comodo AntiVirus - Manual Steps to uninstall Comodo AntiVirus

• ESET Antivirus Removal Tool - How do I uninstall or re-install ESET Smart Security/ESET NOD32 Antivirus?

• Kaspersky Anti-virus Removal Tool - Knowledge base of the Kaspersky Lab

• McAfee Consumer Products Removal Tool - Use on McAfee, AOL distributions of McAfee, CA distributions of McAfee - McAfee Consumer Products Removal tool (MCPR.exe)

• Microsoft OneCare Uninstall Cleanup Tool (All versions; XP & Vista) - Windows Live Onecare Cleanup Tool.exe

• Norton/Symantec Removal Tool - Norton Removal Tool

• Panda Antivirus Uninstall Tools:
  
  • Panda Version 1012 Uninstaller.exe
  • Panda Version 2007 Uninstaller.exe
  • Panda Version 2008 Uninstaller.exe

• PC Tools Antivirus - PC Tools Community Forum - Uninstall PCTools

• TrendMicro Antivirus Removal Tool - How do I remove Trend Micro Internet Security Pro and Trend Micro Internet Security from my computer?

• If necessary for any antivirus or other program try the Windows Installer Cleanup Utility - Description of the Windows Installer CleanUp Utility