Are you using a router to connect to your XBOX to the Internet? If so make sure that your XBOX and PC are connected to the same network.
1. If so on your PC go to Start>Run than type in cmd if using xp if not type in command
2. Once command prompt comes up type in ipconfig/all
3. tell me what your Default Gateway is.
4. The Default gateway on your xbox should be set to what your PC's Default Gateway is.
5. Try FTPing again.
6. If you are using IE try using FlashFXP.
Also did you ever have ftping work between your xbox and pc? I mean before this problem?