Got myself in a bit of a pickle at the moment:
I was toying around with hardware and removed most of the addons in my system, then replaced them. XP decided it was no longer authenticated and asked me to redo the process. It's a somewhat customized system with a basic (possibly OEM, not sure) copy of XP that came with it. I've installed that same disc/key on that exact system once or twice (last time was 2-3 years ago) and it hasn't been registered on any other hardware for a good 5 years.
That particular system usually stays on 24/7 and is not connected to the internet, so I didn't do anything for almost a week. The power went out and it restarted, and now I'm logged out and have to authenticate it before I'm able to log in.
Now, for the annoying part: I use TuneUp utilities (for lack of any decent system cleaners) and used that to disable XP's network support some time ago. As the system is offline except for updates every 6 months, it's a non-issue.
Until now. I can't log in until I authenticate it, I can't authenticate without network access, I can't turn network back on without logging in. I can't authenticate in safe mode. I'd rather not call Microsoft, but that is a potential solution.
I have a few other OSes installed on that system (7 and Ubuntu, which I'm using now), so I can do any tweaks/fixes that are possible from those. I already tried restoring the boot.ini to a pre-TuneUp version, but that didn't help. Network is a wired connection to a router (DHCP on) and from there to a cable modem. The XP authent wizard doesn't see connectivity though, on any settings (since Ubuntu is online, it's just an XP thing).
Edit: And for any of the format-happy folks out there, don't even think about it. I'm looking to fix the install, not nuke it over something this tiny.
Edit2: To emphasize: I'd rather not call. I can, and they won't mind, but I'd rather do it over the internet. Calling is a last resort (for when I have 10 minutes to read the numbers again and again and again over the phone).
Also, I tried again and it told me there was an IP conflict with another system. I think, not sure, but it's possible that the TuneUp backup boot entry does boot with LAN enabled, but I know the DHCP service is off in XP itself. I'm playing with the router itself to see if I can get that working.