Dell OEM XP Pro Activation Problem

I've got an old Latitude D620 notebook which has had Dell OEM Windows XP Pro on it from the get-go, with no problems ever before.

Suddenly, when doing some updates today I got a "This copy of Windows did not pass genuine Windows validation" message out of the Blue, for no reason whatsoever.  It installed all of the updates with no problem but it turned my screen black and continues to nag me with the message.

Of course this many years down the road the Dell OEM CD has been lost & the Microsoft COA sticker has long-since worn off the bottom of the unit, but as I said, this is the OEM Windows which came on it and which has never before had a problem after all the years I've owned it.

When I follow the "activation prompts" it takes me to a Microsoft site which says "this product is no longer for sale" and doesn't even give me a number to activate it over the phone (not that I should need one, because this is the legal original OS which came on the computer).

Anybody know why this suddenly might have happened and / or any work-arounds?  It's ridiculous to hit a roadblock like this after all the years I've had this thing!

XP is so old I can't believe Microsoft even gives a rat's posterior about it now, anyway...

Thanks in advance...

Here is an addendum which I just noticed:  The explanation given says "The product key used to activate Windows on your PC is not authorized for sale or use in the country or geographic area in which you are currently located."  This is even more bizarre since, of course, the product key has not changed since Day One of the installation at the Dell factory.  It came straight from Austin (or Round Rock) to Fort Worth, Texas and has never been anywhere else.  Hope this helps with the mystery.