How do I disable internet traffic on a computer when it is idle?

In the interest of maintaining control of our internet bandwidth usage at home, I’d like to be able to disable internet traffic on a certain windows xp computer which we leave on all the time as a print server. People also use it as a desktop regularly, but it is often idle for large parts of the day. Does anyone know of any software that will let me do this? Ideally it would re-enable internet traffic once the computer is no longer idle. Thanks in advance for your help.