You can still play Halo by temporarily disabling HAC2:
- Right-click on your Halo startup icon (on the desktop or in your Start menu) and select "Open file location". You should see a "controls" folder there.
- In the "controls" folder you'll see "loader.dll". You'll need to move that file out of the "controls" folder (or change the filename to something like "loader.dll.disabled").
Now your Halo should start (but without HAC2).
When HAC2 has been fixed, just reverse the changes you made in order to re-enable HAC2.
EDIT: I see that Btcc22's HAC2 domain has just been renewed. So hopefully the problem will resolved soon now.
EDIT: Btcc22's site is reachable again so your HAC2 should be working now (or it will be soon... depends how long it takes for the DNS entries to update for you).