I realize when and why XP may want to reactivate itself after hardware changes or additions. In my case though, the only change was shutting down the computer one night and starting it up the next morning. (I haven't installed or removed any hardware at all since I built the system over a year ago.)
Something slightly weird happened there, as the system booted up but the graphics card aparently was not feeding signal to the monitors. Hard reset wasn't working, unly unplugging and plugging the power cable back in did the trick. But then, on startup, Windows said it detected a significant change in hardware configuration and needs to be reactivated within three days. My first instinct was maybe something died inside (oh no!), but then again, according to MS, removing or adding a single device should not trigger the reactivation prompt. And thankfully eveything seems to be working fine, I didn't lose a disk or anything like that.
Online reactivation worked and things are seemingly back to normal. I'm still puzzled though, why would Windows do that for no reason? Is it a known ocurrence? Has anyone seen it happen in a similar way?