Seems like a bitmaps.map file problem.. that is where your radar normally is, try reinstalling or changing the graphics settings, but if you don't want to i can send you my bitmaps file, if thats the demo its not too big.
If you dont have the issue with the demo, then its a good chance you got a game disk with some bad or corrupted software and reinstalling won't remove the issue.
If you still have the issue even with the demo, then maybe I'm guessing its a graphics card issue. Do you have one of the new iMacs with graphic issues? Are you running an old mac with a flashed card? Are you running halo for the pc thru bootcamp or parallels? Are you overclocking your GPU? etc...
Some buddies of mine have downloaded the demo and rather than play against total strangers all the time I decided to play with friends. Even if that means playing a Demo. Why they do not purchase the full version is beyond me.