LinuxQuestions.org - Coplicated question (for me!)

Ok, I want to write this program, its a client-server program, and I haven't really ever done any network programming before, that is, except on the profesional app I wrote for Decagon Devices, it just did a version check... =)

Ok here goes:

Client(s):
Will be using Windows or Linux. Linux users could be running Fluxbox, Gnome, KDE, or just about any WM.

Some clients are behind a firewall (which is why I decided to have a server in the mix).

Server:
Will be running RH Linux 9.0

This is what I want the program to do:
I want the program to flag, or notify the other users by sending info to the server, then every amount of time, the clients check to see if there is any updated info. The info will simply be that you are doing something, example, playing a game, and want them to join. I actually just want it to send messages, like "Hello, come join game Bla bla bla! at some ip address", but I want it to notify them by something on the task bar (in windows), or some other way in Linux. Now, I was think of using SDL_Net as the notworking part, because I want it to be cross-platform, however, since I am not a networking guru, I need suggestions. The only compilers I have for Windows are BCC55, the free commandline Borland compiler, and MinGW. I prefer MinGW over Bcc, but suggestions on this would be helpful also. Now, I have the documention for SDL_Net, but like I said, I don't really know how networking works. I will do a google search on this, but I wanted some input from you guys. Any ideas?