i have emulators on mine, i have a gba, n64, nes, snes, and others all working correctly. to do this you must download the correct emulator, try xbins...or xbox-hq.com they are really good.
Also a good place are torrent sites
You can get discs called BAED (Big A$$ Emulator Disc)
each one has like 4GB of roms and emulator, which is about 14,000
games per disc.
I have them on mine so i can say they are safe, although not all emulators work like they should, most do.
First you need the NES emulator installed. I think the NES emulator for XBOX is FCEUltra or something like that.
You put the NES ROMS in the ROMS folder for the NES emulator. Should be in E:\Emulators\name of nes emulator\ROMS or F:\Emulators\name of nes emulator\ROMS
You can get NES ROMS by Google search or bittorrent.