I bought a new Dell computer with a OEM XP Professional. I also have 2 other older PCs and a notebook at home which have XP Home as their operating systems. When I tried to install the new OEM XP Pro on the other older PCs I got a message that the key number is incorrect. Later I have problem using the OEM XP Pro on the new computer that I had purchased. It does not recognised the ID key numbers it came with. Please advise. Khob Khun Ka