Windows Genuine Software

I was over at a client's house the other day.  He had a Sony Vaio laptop and a Dell desktop. Both computers were giving him messages that his copy of windows was invalid.  I know for a fact that he had 2 legit copies of Windows XP, since there were stickers on the Laptop and Desktop.  I went to the Microsoft Genuine Software Website to see what could be done to fix this problem.  Here is what Microsoft had to say:

What do I do if I fail the validation test, but am certain that I purchased/have a genuine copy?

Contact the reseller that you purchased your PC from. If your reseller is unable to help you, contact your local Microsoft call center. The Microsoft call center agent can help you verify whether the Product Key installed on your PC is genuine. The majority of customers who fail validation purchased their PCs from system builders. The PCs came with a genuine COA, but Windows was installed using an invalid Product Key. For customers who have a legal Product Key from the COA or the backup media that came with the PC, the call center agent can point customers to a Product Key Update tool which customers can use to convert their system to genuine, using the legit Product Key they have.

Reading their site a little further, you can find out what causes your PC to appear to invalid.  I think what happened to my client was that he tried reinstalling windows on his PC and accidentally used the laptop's product key.  He has valid software for each, so if he calls Sony and Dell, they should be able to help him out.  The interesting thing is that if Windows detects your operating system is being used on two computers at once,it makes both copies invalid.