The case History:-
A user upgraded his operating system from Windows 98 over a year ago. He activated the installation and has been running ever since without any problem. The otherday when he tried to install SP1 he got a message that his copy of WinXP is illegal. His question -how can it be illegal as he has already activated it and it has been runing for over a year?
Error Message: The Product Key Used to Install Windows Is Invalid http://support.microsoft.com/?kbid=326904
If it does, it means he was sold an illegal copy of the commonly distributed volume license version. He will need to purchase a legal copy before he does anything.