I've triggered it with video card, BIOS updates and more or less!
MS doesn't document this area well.
Because of a problem I was having and discussed in Networking, I needed to disable the onboard NIC on my motherboard and install a separate PCI NIC. (I have since fixed the problem and went back to the onboard NIC.) But here's the strange thing.
I booted to the BIOS and disabled the onboard LAN. Powered off and installed the PCI NIC. I plugged my Ethernet cord into the new NIC and clicked ok. Rebooted and as XP started to load I get this new popup window that said, in essence:
Windows has detected major hardware changes. Windows XP must be reactivated. You have three days to complete this activation.
'Scuse me!? This is before XP had fully booted.
I plugged in the Ethernet cord into the new NIC and clicked ok. Now another window saying XP could not establish a connection. After two more "Try again" clicks, I canceled it and let XP fully boot.
After a full boot, and the discovery of the new NIC, I go to Start, Programs, Accessories, System Tools, and clicked on Activate Windows. It does in an instant with my cable connection.
Why would changing the NIC constitute a major hardware change?
Everything is hunky-dory now, just thought I'd share this.