After one too many false alarms, system slowdowns and hopelessly phrased help screens, I decided to ditch my installation of Trend Micro Internet Security Pro and switch to CA's Security Center. I knew that would involve much uninstalling, installing and rebooting. However, I forgot that in the brief (and offline) period when I'd dumped Trend and hadn't installed CA, Vista would reactivate its built-in Defender software, creating many a potential conflict down the road if I didn't banish it from my system start-up.
To disable Defender, run it from the main Start menu, click on Tools, select Options, scroll down and untick 'Use Windows Defender' under the administrator section. Click Save and fight your way through the UAC prompts. Double-check that it's been exited from the task bar as well, and you should be good to go. If Microsoft pushes ahead with plans to provide free security software for Windows users, this process will likely get even more fiddly in the future -- but running multiple security software packages is pretty much always a recipe for disaster.