If the printer is configured for printer sharing on the network, the Windows PCs should be able to find it, if the printer has been set up for them. You say that your Mac Pro machines are able to print through one of your iMacs. This implies that this one iMac has printer sharing enabled in System Preferences. Can't you go into the Windows machines' preferences and find the printer through the iMac? Or at least, over the network in general? It shouldn't be too hard. My (now dysfunctional) PC had no problem detecting my network HP all-in-one printer. Check out printers in the Control Panel is my guess.
I have a network of 4 macpros, 6macbook pros, 2 imacs & 7 windows machines running xp, and a laptop running vista business. I recently purchased an airport express base station to put my usb printer (Epson Stylus photo 1390)on the network. All macbook pros & imacs are printing efficiently on wireless. Macpros are using the printer shared from one imac. But i don't understand how do i connect my windows pcs to the printer (all are on ethernet lan). Is there a way to connect them. Please help.