Hello.
I know that's a long answer to this question, but a link to where I can get dosc on the subject will be anough.

Can I connect directly to any PC that's connected to Internet via MSComm or INet or something?

I mean: let's say I and a friend of my have installed a copy of "my-personal-chat-prog".

Can I check if he's online and begin a two-way communication between the two PCs by calling him (obviously he's got no registered domain, all I can know is his ISP)?

Thanks anybody!