http://technet.microsoft.com/en-us/libr ... 57054.aspx
... including what hardware is checked, and how many changes are required to trigger an activation.
Installations of Windows XP made using volume licensing media and volume license product keys (VLKs) will have no activation...
Microsoft provides OEMs with the ability to "pre-activate" Windows XP in the factory ... "System Locked Pre-installation," or SLP ... At boot, Windows XP compares the PC's BIOS to the SLP information. If it matches, no activation is required.
Every single piece of hardware could be changed on a PC with SLP and no reactivation would be required — even the motherboard could be replaced as long as the replacement motherboard was original equipment manufactured by the OEM and retained the proper BIOS.
Vista (not so much technical info on this...):The hardware hash is an eight byte value that is created by running 10 different pieces of information from the PC's hardware components...
http://www.microsoft.com/uk/windows/pro ... onfaq.mspx
Windows Vista uses an algorithm to determine when changes to hardware are significant enough in nature to require re-activation. The algorithm used by Windows Vista is more flexible than that used by Windows XP, but each component change has an impact and adds up until you reach a pre-determined limit. ... Most users will find that Windows Vista activation is similar to activation in Windows XP, but easier