Windows doesn't natively support Bonjour. You are probably finding the printers through Wins/Netbios over network neighborhood.
Apple does supply a Bonjour driver for Windows. Go here:
http://www.apple.com/support/bonjour/ to get the downloads for Bonjour for Windows and support for it.
Make sure that the printer network configuration is set up to provide Bonjour transmissions. HP is primarily a Windows shop, and although they provide OSX drivers, not much of the hardware provides for Bonjour. If the printer doesn't support it, then you will have to set up the macbook for IP printing and the printer will need to have a static IP address.
Remember that printer sharing from an iMac is not the same as using a printer in a stand alone Bonjour environment. In this case, the iMac is providing the Bonjour information.