When installing Windows there are 4 possibilities.
1. Buy a Key.
2. If you are re-installing/installing Windows and you have an OEM version of Windows, that copy will only work on the motherboard originally installed on.
3. If you re-installing/installing Windows and you have a Retail version, you can install Windows on I believe 3 different computers.
4. Don't install windows.
I do apologize for the answer I gave in 2 parts. I do agree with the others who posted replies to the person's query. I have a friend who installed XP program to his computer, that he'd bought from a local computer store, and he was told by Mircrosoft that it was not genuine .I advised him to return the program disk for a refund.
i have been told that you should be careful not tobuy any windows programs from EBay as they usually are not genuine.