How do you create a Yahoo chat client?

Question

I finally got my own chat application working across the internet. Which means I configured my router to allow port forwarding, opened the right ports on windows firewall, installed and configured IIS7, bought a domain so the IP address on my computer can be resolved and I can run a server program that connects people, and setup a program that updates DNS when my IP addres changes. I feel like I have a good knoweldge base and I would like to take it to the next level by tapping into Yahoo Messenger. I don't really know where to start and any advice would be very much appreciated.