Calling all Windows Genuine (Dis)Advantage experts

A while ago, I picked up a cheap PC that came pre-loaded with Windows XP Pro. I was going to use it for testing infections outside of a virtual environment. I have a machine for this already, but a backup is always good, right? Imagine my dismay when, a few weeks into using it, the dreaded OH NOES THIS IS TEH FAKE WINDOWS messages started appearing. Nag screens at bootup - nag screens while using it....nag, nag, nag. I decided to hose the thing and install Linux instead, but thought I'd best get the few pieces of data from it that I'd left there. When I booted it up, I was surprised to find......no nags. Nothing. Even more strangely, it was now happily installing all those banned updates that previously didn't want to play.

Can someone explain what's happened here? Did the WGA thing screw up? Why was I now being told my Windows copy was legit? I thought this thing didn't make mistakes? Ultimately, it's all academic as the PC fried itself and is now sitting at the bottom of a landfill somewhere, but still. Weirdness.