Make sure your Xbox 360 region is the same region as the original Xbox games that you have. You need to buy the Xbox 360 Premium set (the one with the hard disk), by default it should have the emulator software installed but if you can log on to Xbox Live you will automatically download the latest emulator when you pop in your original Xbox game into the 360.

Likewise to play your orginal Xbox game on the 360, just eject, pop in the disc, close the drive and BAM! You're good to go!....well at least for those games that in the backward compatibility list.