o.O...
Well windows for 1 are the best way from getting rid of *rubber rockets*...
If you like to play in full screen and hate a window try this...it is a window basically Full screen...it works (*cough* MaryK8, Elvis *cough*)
1) Right click on Halo, properties.
2) Where it has target, Press space bar and -window (this should basically be where youve put -console)
3) Get into Halo in the window (make sure its a window) press the window key (bottom right between control and alt or near the arrow keys) Click on the blue bar on the outside up the top (like you do to perhaps drag vent around the desktop) and drag to 1 of the corners (e.g. top left or right)
4) It should have re-entered halo now, so press the windows key again.
5) Click on the THIN blue bar on 1 of the bottom corners and drag it to the outside covering the whole screen.
6) DONE, a window in full screen. It SHOULD help your lag.
Remember with ^^ make sure your load the game from the icon or whereever you applied the command (like Xfire)
If you need screenies just ask Via PM and ill post them.
Also to increade FPS Instead of having in video options the FPS at 30FPS, try VYSNC or No VYSNC. ( think thats the spelling)
Hope it helps mate, GL.