The actual algorithm that Microsoft uses is not disclosed, but we do know the weighting of components is as follows, from highest to lowest:
- Motherboard (and CPU)
- Hard drive
- Network interface card (NIC)
- Graphics card
If you just add a new hard disk or add new RAM, there is no issue. If you create an image of your Windows 7 installation on another hard disk and swap that hard disk into the system and boot from it, or if you replace all your RAM and reboot, WAT gets triggered and checks to see whether you must reactivate Windows 7.
In theory, chances that you'll get stung by any of this are not great. It was widely expected that the only users who'd need to worry about reactivation would be users who'd buy a preinstalled system, image the hard disk or try to move the hard disk to a newer, faster computer, or perform a motherboard upgrade using a preinstalled copy of Windows 7.
Unfortunately, in practice users have been forced to reactivate after relatively modest hardware changes. In one Vista example, a user who changed from a DirectX 9 -- to a DirectX 10 -- compatible graphics card had to reactivate his installation. But wait, it got worse: Another Vista user had to reactivate Windows after upgrading to a newer version of the Intel Matrix Storage driver for his motherboard. Essentially, WGA mistook a driver upgrade for a significant hardware upgrade. Users who missed the three-day reactivation window (it's easy to do) found themselves needing to make a phone call to reactivate. Users who were hearing-impaired found that difficult to do.