You can earn Microsoft points by purchasing Halo related content and by playing multiplayer if you're signed up to the Xbox Live Rewards program.
How does it all work? If you play multiplayer for 35 or more hours you'll get 100 Points, play for 70 or more hours and you get 300 Points, or if you play for 140 or more hours you will get 600 Points.
That isn't the only way to earn points, you can also earn them by purchasing downloadable content. If you spend points on Halo Game Add-Ons, Avatar Items, Themes, and certain Games on Demand you will be rewarded.
Spend 1500 or more Points, and get 100 Points back, or Spend 3000 or more Points, and get 200 Points back.
You can earn a total of 800 points, although you'll need to hurry as this offer ends on 30th November, 2012.