Alright man let me break it down for you... This is how I do it.
(back ground: I work IT and I use RDP and VNC on a daily basis to controll everything from p2's to brand new Xeon Servers.
At work I am really not supposed to on aim, Irc etc etc.. So I set up an RDP connection, now I also have wireless for inet and a nic card for cat 5 connection to my xbox. So this is a 2 nic set up)
Ok first off on my pc I enabled remote desktop and created a dummy account on the pc for my remote access for RDP you enable port forwarding to port 3889 to the pc on the network.
Now you can use vnc but you will need a vnc server on his pc, have to forward those ports (diff from rdp ports) and then have a vnc viewer installed on your pc.
I really do preffer using rdp casue i can manually map my local C: here at work to my home pc.
the xbox needs to be on and the nic card set up with a static ip on the xbox and the pc nic you use to connect to your xbox....
Now rdp into his machine.
Start up Flashfxp, or some other assorted client,
Ftp into the box
Start your transfers.
Now understnad if you are transferring something from your local c: over the internet into his xbox it will run much slower than if that drive was in his local computer...
Now You can sit in your underwear smoking a phattie while accessing his box and ftping in the nonsense he needs.
Excuse my horrid typing..
Edit: something you need to understand is that when you RDP into his machine he will not be able to watch what youare doing it will appear as if the machine is locked or not logged in from his side.
Also RDP is built into all XP machines, so no files to install. and it is relatively easy to install holler back at me on here for a more indepth means to set this shit up.