Windows XP Professional Upgrade License

I am considering upgrading my Dell L550 to Windows XP Professional, as many of the programs (e.g., McAfee, no matter what the box says) and periphials (e.g., MP3 players) out now are not compatible with 98 SE. However, the Dell is getting rather old, and I am concerned that I will need to get a new box in a year or two. My question is: If I do upgrade my computer, and something happens and I must buy a new box, will I be able to save money by getting a computer that comes preloaded with XP Home, and just upgrade it with the XP Professional Upgrade, or will Microsoft not allow a copy of XP Pro upgrade that has already been installed on one machine be installed on another?