Ok, this is my first time submitting a cheat but its a good one.
They say there are no cheats but there is/
To get all levels follow this method:
1.Open up Halo and create a new profile.
2.Open up my computer and find my documents, for Windows XP it should be the same.
3.Find My Games and then click Halo.
4.Click on save games and find your new profile.
5.Then under checkpoints create a new text file called checkpoint0.sav
6.In it put:
0,0,0
0,0,0
0,0,0
7.Create another one called checkpoint1.sav and put
1,0,0
0,0,0
0,0,0
The 1 is off of checkpoint1.sav and you will progress to checkpoint9.sav changing the first number as you go.
8. Do this untill you get to checkpoint9.sav
9.Now use your new profile in campain and select load game.
10.Give yourself a pat on the back if it woked!