1. Number must match type of system
i.e XP pro - XP home - XP media centre
2. Number must match Retail or OEM
3. Number must match patch version
ie. no sevice patch
XP with SP1 - XP with SP2 as far as I'm aware there are no Install Cds of XP Sp3 (but I may well be corrected)
4. Number must match batch version
i.e. XP-Home with SP2 has several batches of CDs issued
1 you can't really get round must be correct version
2. if you have the number for the disk install using that number and then edit the registry to suit the number on the machine sticker and phone for reactivation
3. Install older version i.e. sp1 instead of sp2
using original disk number, update service pack, edit registry and phone for reactivation
4. as per 2
I have probably done all of these at one time or another when repairing machines for people who have let their PCs get so bably infeste that the only thing left is a format and reinstall.
Biggest problem is Certain manufactures like Packard bell that "tattoo" their hard drives / motherboards / BIOS to the installed Operating system so if you change HDD motherboard you then seem unable to reinstall or even use the original operating system number.