A reporter from APCMag found out that something as simple as swapping a video card or updating drivers can trigger a deactivation of Windows Vista. When this happens your operating system will enter reduced functionality mode and to activate it again you'll 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.
On their own, swapping a video card or updating a driver isn't a problem but cumulatively they can cause Windows Vista deactivation.