After weeks of gruelling troubleshooting, I've finally had it confirmed by Microsoft Australia and USA -- something as small as swapping the video card or updating a device driver can trigger a total Vista deactivation.
Put simply, your copy of Windows will stop working with very little notice (three days) and your PC will go into "reduced functionality" mode, where you can't do anything but use the web browser for half an hour.

You'll then need to reapply to Microsoft to get a new activation code.

How can this crazy situation occur? Read on for the sorry tale.