For Xbox owners who didn't buy an Xbox 360 yet, this might be a good time to move on.
Current paid Xbox LIVE member using an original Xbox console (not Xbox 360) who purchase an Xbox 360 between November 21 and December 21, 2007 and move their existing paid Xbox LIVE account to an Xbox LIVE Gold subscription will have Microsoft sending them a copy of Halo 3.
To get your free Halo 3 copy, all you have to do is upgrade your existing Xbox LIVE subscription within the specified time frame and register on the official Xbox website (link available in the downloads tab).
Step by step guide :
1 - Buy an Xbox 360 console between November 21 and December 21, 2007.
2 - Convert your current paid Xbox LIVE account to an Xbox LIVE Gold subscription.
3 - Register here between November 21 and December 21, 2007 with promotional code XBX7777, with your new console serial number, and a valid mailing address.
4 - Look for your copy of Halo 3 in your mailbox. It will arrive by January 18, 2008.