Changing the mobo changed the system id, so it basically is a new computer for licensing purposes. I'm assuming the videos are in MTS streams or similar. You might be able to try VLC, FFMPEG, or another decoder/player combo like WMPClassic and FDDSHOW to see if it'll ignore the flag. Beyond that, uh, assuming it's just the flag that's in your way you could probably remove it with a hex editor or something. Maybe re-encoding with software that ignores the flag?
But, again, this is just kinda random guessing. I've never really used MTS/M2TS except for a random playback here and there. So long as there isn't any encryption/scrambling then I assume all you need is some software that just ignores the flag.
Anyway, if none of that helps or I'm off-base, then maybe the bump will get someone's attention that's more knowledgable 😛