Xbox is one of the most difficult consoles to create an emulator for, as its specs are demanding. The biggest problem for gamers hasn't been the system requirements though but the lack of an appropriate program to run commercial games, using their ISO file. "Xeon" is the most reliable emulator, capable of running games you find on stores, even though its level of support is still low.
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.
Keep searching for new updates of the emulator. Halo was the first commercial game to be run by Xeon, but the list keeps expanding with newer updates and patches. When using the emulator, close any unnecessary applications, to make full use of your CPU's capabilities.
Always check the files you download for viruses. Emulators and ISO files are not official Microsoft products, so be wary of possible dangers.