Well, you could try hitting the website to download the latest patch. In the end I had to go to the game folder (C:\Program Files\Microsoft Games\Halo 2) and run Halo2 as an admin and then it updated for me.
If that method of update doesn't work immediately, play around under the LIVE account area and I think you can force an update from there. I can't remember exactly where I did it.
But... the same fix that worked for me worked for you... so this gives Gilgamesh two seperate machines showing two different errors both dealing with the WinVista profile.