Using the software's uninstall program, or Windows Add/Remove Programs is usually suficient, but even that often tends to leave pieces floating around. It's best to be sure get it all out before installing your new software. There's always the chance that the new doesn't like the old and they start fighting each other. Most anti-virus producers realize this and have written their own cleaning software, or have posted instructions on where to find these last pieces and remove them. Here are some links to the more popular anti-virus software programs. Just click on the appropriate name for the link. If there are no specific instructions given on how to uninstall the software, I would run the uninstall program that came with it, reboot and then run the cleaner.
AvastAVGbitdefenderComputer AssociatesESETF-SecureKasperskyMcAfee (I can no longer find this on the McAfee site)Panda SecuritySymantec (Norton)Trend MicroZoneAlarmIf you're using an anti-virus program that is not listed here and don't know if there's a special removal tool for it, feel free to ask and we'll be more than happy to search with you.
