Opening Socket Port - Danger Of Malware Infection?

My current project is a program that collects meteorolgical data on one computer and allows to access it from any other computer on the net.Everything is working fine so far - it is collecting the data, doing some computation and then listening (as server, TCP) on some port for a client who is interested in downloading this data.When it receives some short command string (something like "GET DATA") it just starts sending the data to the client. The transmission works.

At this point I am getting a bit nervous because I absolutely have no idea on the programming basis of viruses and other malware:When the program is listening on the port (for which I have to allow an exception in the firewall) is there any danger that malware from outside intrudes into the computer?(running Windows XP with latest service packs installed, firewall enabled, antivirus-program installed (is this necessary if there is absolutely no internet activity on the computer besides of this data sampling / server program?)).

If there is danger, what could I do to protect the computer aside from closing the port which I need for communication with the external world?Can you point me to some URL where I can alleviate my ignorance of the basics of malware functioning (on the software level)?