Messenger Program

Hello,

i thought about programing a messenger program, similar to msn or icq.

There should be a server where the clients connect to and authenticate with a user id and password.

The communication between the different users (2+) should run over the server.

The user information should be stored in a Microsoft SQL Server database.
(Passowrd,Usernam,ID,...)
In a second database the messages "to offline users" should be stored.
 
That should be enough for now...

Which network component should I use (Delphi 6 compatible)?
Shall I work with winsocks?
Which protocol should I use?
Do I have to work with Threads if i want to send messages to several people at the same time (client and server)?

It should be possible, that several users from one network with only one internet ip, can use the messenger program as well.
How should i manage all the different connections and users on the server so that each messages find the right way to each other.

Thanks for your tips n tricks!
Marc