windows xp promt for activation, but already activated

Hi,

My company have a few hundreds of Kiosk with operating system windows
xp sp2.

The Automatic Update is on. All the Kiosk cannot access internet, only
LAN. So effectively cannot automaticly update itself.

All the XP have been activated before. Running for a few months
without problem.

Then sometimes some computers would pop up a message
"This copy of Windows must be activated with Microsoft before you can
log on".


Why is the problem occur? Any method to fix it?

Thanks.

Regards,
Bill