There is a patch for the full version that stops that error from showing. It
is a DirectX check, but as noted, DX10 is backwards compatible. Halo does
run, as long as the check is not performed.
I got the patch when I connected to bungie.net as an MP game...does the demo
allow you to connect to bungie.net? maybe there is a patch for teh demo in
their forums...though I doubt it...I imagine they feel if you did not buy
Halo yet, you aren't going to.... ;-)