The iPad uses Apple's printing protocol AirPrint. This is not a feature of Bonjour Print Services since it only lets it find printers shared by Macs and share it's own. There is however a quick and easy third party Windows application which can advertise a Windows printer using the AirPrint protocol.
It does require the windows computer to be on and the since the printer won't be advertised if it isn't. If you have problems just google "AirPrint Windows" and you'll find that page and another tutorial.
Again there is another third party app you can use:
I've tried them both and they work a treat for me.
Hope that's helped! I finally signed up because I actually knew the answer to this question! Yay!