Instructions
Go to the Emulator Zone, at emulator-zone.com. From the main page, click on the "Microsoft Xbox" link, at the left end of the screen.
Choose the Xeon emulator from the list. Xbox's specs are listed on this page as well, in case you want to compare its capabilities with your PC's.
Choose Xeon 1.0 from the list. It is the only emulator running commercial games, with Halo's NTSC version being the first of them. Your system requirements, in order to run a game properly, are also mentioned on this page, so make sure your hardware can support the emulator.
Download the file. It will be compressed, so make sure you create a new folder, before you attempt to decompress it. You don't want to lose any of its ".dll" files.
To run the ISO games, simply drag them on the application, or use the "Open" option of the program.