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.
Just over a month ago I swapped over the graphics card on my Vista Ultimate box. There were some new DirectX 10-based titles out and I couldn"t get the benefit on my old DirectX 9 card. The swap-over went well and I went on my merry gaming way.
Then a few days ago I got a Windows Activation prompt – I had three days to activate Windows or I"d be bumped back to RFM (Reduced Functionality Mode). What the? My copy of Vista was activated, and a graphics card change shouldn"t have triggered deactivation... surely!
I was able to reactivate easily enough, although as the product key was already in use (by me!) I couldn"t reactivate automatically, but had to speak to a Microsoft customer service representative.