I just started out in Ubuntu and
have been asking lots of questions :-) I hope I can give you a good
answer. I haven't done too much work with Vista but printer sharing
should be the same as XP. It helps if the Ubuntu machine has a
permanent IP address. I made mine 192.168.1.5. In printer
configuration under server check the settings you want. I selected Show
printers shared by other systems, Publish shared printers and Allow
users to cancel any job. Under the printer properties check shared.

On your windows machine I had to install the printer to load the drivers
because it wouldn't show my model. Just temporarily connect the printer
and using the cd that came with the printer do an install. Then connect
the printer back to the Ubuntu machine and do another add printer on the
windows machine. Check connect to a printer on the Internet or on a
home network and in the URL entry enter the direct address of the
printer including the ip address of the Ubuntu machine, the word
printers and the printer name. My example is:

http://192.168.1.5:631/printers/ml-20102

Hopefully this works on your Vista machine. If I can't get the win 2000
machine to work I may have to talk my son into installing Ubuntu. He's
not as geeky as I am so he may not go along with it