Typically all bit-fields with the exception of the unused field and the 'dockable' field are compared. If more than three of these ten bit-fields have changed in a) since product activation, re-activation is required.I really would encourage you all to read through the whole thing, though. If this does spell out exactly what the XP activation scheme's limits are. It's not nearly as bad as a lot of people seem to think it is. It's still activation, which in itself isn't appealing. However, if this account is correct, XP's activation looks to be somewhat less of a pain than many had originally thought.
This means, for example, that in our above real-world example, we could replace the harddrive and the CD-ROM drive and substantially upgrade our RAM without having to re-activate our Windows XP installation.
However, if we completely re-installed Windows XP, the information in b) would be lost and we would have to re-activate our installation, even if we had not changed our hardware.