[rajas]

Extract from EULA of Windows Vista Ultimate..

INSTALLATION AND USE RIGHTS. Before you use the software under a license, you must assign
that license to one device (physical hardware system). That device is the “licensed device.” A
hardware partition or blade is considered to be a separate device

the rest is almost same as windows XP blah.blah..blah...Read below for some info on XP product activation and its tolerance.

How does product activation determine tolerance? In other words,
how many components of the PC must change before I am required to reactivate?

Common changes to hardware such as upgrading a video card, adding a second hard disk drive, adding RAM or upgrading a CD-ROM device will not require the system to be reactivated.

Specifically, product activation determines tolerance through a voting mechanism. There are 10 hardware characteristics used in creating the hardware hash. Each characteristic is worth one vote, except the network card which is worth three votes. When thinking of tolerance, it's easiest to think about what has not changed instead of what has changed. When the current hardware hash is compared to the original hardware hash, there must be 7 or more matching points for the two hardware hashes to be considered in tolerance. If the network card is the same, then only 4 additional characteristics must match (because the network card is worth 3, for a total of 7). If the network card is not the same, then a total of 7 characteristics other than the network card must be the same. If the device is a laptop (specifically a dockable device), additional tolerance is allotted and there need be only 4 or more matching points. Therefore, if the device is dockable and the network card is the same, only one other characteristic must be the same for a total vote of 4. If the device is dockable and the network card is not the same, then a total of 4 characteristics other than the network card must be the same.

Are the changes cumulative? In other words, if I change one component today and one tomorrow, is that two component changes?

The changes are cumulative; however, if a user is asked to reactivate, the hardware profile is reset to that new configuration.

read more about the same here: Product Activation Facts