When Vista pegs it as completely new hardware. APCMag has a story on how Vista's activation scheme can suddenly deactivate your PC and give you a nasty surprise:

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. I got the code easily enough, but it didn't explain why Vista had deactivated, so I got in touch with Microsoft about the problem.

They sent me some special utilities to run which gathered the history of hardware changes on that machine since activation, and it turns out that my disk controller had changed, so the graphics card change was the final change which tripped deactivation. The only problem? I had never changed my disk controller at any point. Apparently because I had upgraded the Intel Matrix Storage Manager application, this was reported as a major hardware change event.