Get WinAmp. It plays it out of the box. No need to download any codecs. I don't exactly know what codec this uses. Just whatever Ulead Video Studio encodes to. If you're really desperate, you could get a video converter and convert it to Quicktime's MOV format. But, I don't think it's worth it for a 1 min 27 second fan-made Halo 3 trailer.
Ugh, I am so sick of the people on the Bungie Forums. Bunch of fanboy dumba****. "The music sucks", "You need a guitar solo", "What's up with Armageddon???". That's all I hear from them. It's like nothing if ever good enough. And this is my first try at ever making a video like this! I'm not freaking Marty, people! (The guy who composed all of Halo's music).
Oh well... they don't know anything anyways. They're all crying over the loss of their wonderful "clanmatches". Big deal...
Anyway, sorry about the codec thing. I would upload it in another format. But the other formats are too large. It took me 1 hour and 30 minutes to upload this one on dialup... and to think, all of that trouble to get flamed by a bunch of fanboys. *sigh* Why, oh why, do I even bother? (I'm not talking about the people here, btw.)