My client has asked me to do a feasibility study on transferring files between 2 PC via the internet.

I would like your input on:
- What is the best method to transfer files? (E-mail, FTP, etc.)
- What tool is best suited for this task? (ActiveX from MS, 3-party tool, etc.)
- When one PC receive a file, it may have some file to send to the sender. How it can be implemented?
- How an application can retreive the files when it's received?
- Hardware consideration?
- This fonctionality must be implemented as a COM to be integrated in other applications. Any gotchas?

TIA