Direct Transfer Programs?

I've been using AIM to transfer files to another person, that or set up my own FTP server.

But is there some simple Microsoft command line program that sends a file to another computer?

If so, how would you specify the recieving computer if that computer is behind a home router and shares its public IP with other computers?

Can you specify the client computer to send to without having to ask the client to go into his router and change the port forward settings?

Stick with FTP. The router can be set to pass the connection on. FTP is also built into the command prompt, just type ftp.