Halo's prolly barfin' chunks at the unknown resolution it's being asked to deal with on the new setup. (Both the new size/aspect ratio, AND the extended desktop, all at once.)
Strangely enough, in this case I'm going to suggest UNinstalling Halo and REinstalling it so that it can "start fresh" and maybe better figure out your setup. (Don't worry, it will keep your profiles... which might keep this from working tho! So once you've UNinstalled, rename the "Halo" folder in "My Documents\My Games" to something else BEFORE you install. Then after that works, exit Halo and rename it back and try it again to see that it still works.)
BTW: Halo has a hard time with certain aspect ratio screen resolutions, so you might have to try some others as well....
Oh, but first!: If that was me having this problem I'd make the problem happen again like a hundred more times, and each time tell it to "Send results to Microsoft to help diagnose the problem" so you can "vote" on how crappy their OWN SOFTWARE IS!