When Microsoft launched Windows XP, users had to validate the software serial number with Microsoft to ensure the software was genuine, and had not been installed on multiple machines.
To protect the software from being installed on other machines, Microsoft added a logged scoring system, where by changes to the PC were given a points value (e.g. change of processor 4 points, change of hard disk drive 3 points etc). Once this points total reached a preset value, Microsoft considered this a new PC, and hence an invalid system install. It could be that your XP version is genuine, but sufficient changes have been made to the machine since initial install to cause the problem.
Microsoft will not do a lot to help you if you find yourself in this position � other than to sell you another copy of their software.