Pure Baloney

It's trivial to install Linux on the AppleTV. Several Hundred over at xbmc have do just this. Many more have installed CrystalHD cards and are enjoying 1080p video content playback including decrypted Bluray.

Windows, Heheh. I think I could get WinXP to boot if I could ever find the time to finish that project.

OSX 10.4.7, yep can do that.
OSX 10.5.x, I've not done it but others have.
OSX 10.6.x, I agree. Waste of time.

Now browsing internet in desperate hope to recover original ATV OS, which is also next to impossible
What, your google-fu must really suck bad. All versions are out there. You just have to search with good keywords.