Actually, all I did was deselect the IPv6 on each of the Vista machines (no such selection...that I am aware of for XP). I opened the network connections properties for each of the Vista machines (I did not do this for Windows 7 as it was able to connect all along) and disabled IPv6. Once I did that, I was able to connect to the printers on the XP machine.
Now please understand, Nigel probably is 100% correct...it may be just a total coincidence that this worked for me...I had tried everything else that has been suggested in so many places with no success. This was the last action I took before I was able to print to the printers from Vista based systems. As I said in my earlier post, it may have nothing to do with your issue, but it seems to have worked for me.
My Best Always,