HBO's own Louis Wu posted a note that IGN has put up a complete-- and I do mean COMPLETE-- walkthrough of Halo. As Louis notes, you can get around having to register with IGN to see it if you turn off javascript in your browser. Here's what they say in the introduction:

Halo is the best first person shooter ever made. It's not the first, it's not the most influential, it's simply the best. The combination of great control, amazing story, excellent enemy AI and slick level design adds up to a truly impressive game indeed. Is it the best launch game ever for a system? It would be hard to argue with Super Mario Bros. in this category, but Halo will have you rethinking this. It will have you rethinking a lot of things. Like which console you're going to buy this holiday season. Like whether or not you need to eat. Or sleep. Or do anything you should be doing. The single player game is a masterpiece, and the multi-player will keep you and fifteen of your closest friends warm for years to come.

If you don't want to know too much about the game before you play, don't read any further than that.

UPDATE: Harry pointed out that the walkthrough is detailed, but not complete; it only covers the first three missions or so.