Most people would know better than me, but I think you can change it "manually" in target line of your shortcut (and I guess xfire as well). The internet says:
Right-click your shortcut and go to Properties
On the Target line, add -vidmode height,width,refresh rate
"...\Halo\halo.exe" -vidmode 1920,1080,60The internet has also said that some people have trouble with their chat bar at the bottom of the screen when doing that, and some other problems.
Probably make a new, temporary Halo shortcut somewhere and try it on that first. I learned that from Mxy