I did as you advised, and uninstalled and reinstalled the software for the Canon Pixma mp780. The laptop with XP recognizes and finds the printer, but when I ask it to print, it makes the attempt and comes back with the message "failed to print".
I have used this printer on the laptop for a couple years now, and it just pulled this stunt after I purchased a new desktop with VISTA and had to find a new driver for it.
I have Vista Home Premium 64bit.