Step 1: FTP and what it's all about and how it can help you!
A Crossover Cable (cheap and can be found at most electronics stores or made if you have the gear)
An FTP Program (i use Flash FXP but whatever flavor you like works)
A modded XBox (mine is softmodded with the tutorial that i published)
If you have XBMC installed as your Dashboard, you can find out your XBox IP address in the settings section (once you have connected your XBox to a PC via Ethernet with a Crossover cable). After you set it to be online (not hard trust me.. if you soft modded your XBox you can do this part yourself). Then simply open your FTP program, then in the section that allows you to set a connection, simply enter the IP address from the XBox (write it down and stick it on a post it on your computer), then enter in 'xbox' for username and password (that is the default UN and Pass for the Xbox). then its a simple matter of transferring the files from the directory on your computer they are saved on to the are on your PC to your XBox they go to usually on the E Drive (under the Dashboard folder for softmods). Then you can load the files in the corresponding subfolders (usually under Plugins, there are ones like Video, Audio, Programs, Emulators etc). You can usually find some of these folders in forums and with simple Google Searches.
Step 2: Sharing Media with your PC
Note: XBMC for Xbox will play most any format you throw at it, but High Definition Video under the H.264 container is pretty choppy and problematic due to the memory limitations of the Xbox.
Step 3: Playing against others using System Link Games!
start the game and then go to the System link screen (and log into your profile, like in Halo 2), then simply go on your client and then under Tools in the menu, have it detect your console. after that all you have to do is click the room of your choice under the game you are playing (staying in System Link mode on the XBox) and you can still play against people from around the world. You can even use your Xbox Live Communicator Headset on there to talk smack to your opponents. but i do no condone teabagging! One interesting note is that many from South America frequent this system as Xbox Live is not avaliable there.
Note: you do NOT need a modded system to play this, but you CAN use backups to play on here (i tested with a backup of Halo 2)