I know this is an old topic, but I just thought I would give my answer.
I have a Windows 7 PC, and had a printer hooked up to it via USB. The PC is hooked to our router, and so are our two other computers.
My second computer is a PC running windows XP.
My third computer is a Mac.
I 'shared' our printer on our win 7, and so I could access it on our XP, but not on our mac. After hours of going through forum and forums trying every single thing that everyone said to do, I called Apple Support.
They told me that sharing a printer for a windows 7 to a mac wasn't they're problem and they said to contact my printer manufacturer. So I called HP, they told me that they couldn't do anything for me and that all I had to do was buy a wireless printer.
So I came I ended up coming to this conclusion:
Solution #1 - Buy an Apple AirPort router (it has a special printer share usb plug) (100$)
Solution #2 - Buy a wireless printer (ranges between 200$ - 400$)
Solution #3 - (what I did) (0$) Plug your printer via USB to your mac. Sharing with the windows computer then takes 30 seconds to set up. Just open Control Panel>Printers>Add printer (one of the buttons on the top). Then follow the onscreen instructions , Select network printer, (then it shows the computers on the network), select the one that you want, click next, install the appropriate driver, and BINGO! its done.
(This is assuming that all of your computers are already hooked up to your network)
Anyways,
Hope this helped,
PCgeek