Okay first and foremost I believe the best way is to uninstall and reinstall if you still have your CD.
But if you're okay with missing out on the slick sniper and infiltration level here's another solution as long as you have the PC version.
To access all of the levels in the game, you have to mess with your game directory, which could cause problems with your game or cause other problems. Therefore, attempt this at your own risk.
* 1. Create a new profile
* 2. From Windows, locate the 'checkpoints' directory for the newly created profile. You will find this in the My Documents folder (such as D:\Documents and Settings\...\My Documents\My Games\Halo\savegames\All_Levels\checkpoints)
* 3. In this directory create a text file called checkpoint0.sav with the following text included: 0,0,0 0,0,0 0,0,0
* 4. Repeat the previous step until you have ten files named checkpoint0.sav ... checkpoint9.sav and for each file change the first number on the first line to indicate the level (such as 0 The Pillar of Autumn)
* 1 - Halo
* 2 - The Truth and Reconciliation
* 3 - The Silent Cartographer
* 4 - Assault on the Control Room
* 5 - 343 Guilty Spark
* 6 - The Library
* 7 - Two Betrayals
* 8 - Keyes
* 9 - The Maw
So, for instance, for 'The Maw' level, you will have a file called checkpoint9.sav with the following text:
* 9,0,0
* 0,0,0
* 0,0,0
* 5. Now, select Campaign and Load Game. You should now have access to all the main game levels.
