Be sure to connect the printer to your network, then it should be detected as AirPrint, printer sharing will not allow using AirPrint.
If I correctly understood your intention by your title, you have the printer connected to an XP machine and shared through the network?
If I misunderstood your intention please clarify further.
You may find the guidelines to use AirPrint in the following document:
If the printer connected to your router and still cannot be located please clarify your router brand and model.