you need a crossover cable .. you can make one from a patch cable or just buy the M$ system link cable.
Now you need to download the xbox emulators from 'the usual places' because pc emulators will not work on the xbox. Rom's that you have on you comp, however, will work.
Then you establish an FTP connection to your xbox, transfer the roms and emu's to you hdd, reboot the xbox and they should show up on the dash if its configured (mine was).
This is as far as I can help, I think, there's other people way more knowledgable in this than I am