The Lexmark site Z640 driver download offers this warning that may be of interest...
"Attention Windows Vista Users: Attention Windows Vista Users
If you have installed your printer on your PC using an original Lexmark Driver CD that came with your product and you are experiencing problems when printing, you may have installed the Windows XP driver.
Follow these steps to resolve:
· Disconnect your USB cable from your printer.
· Uninstall all Lexmark printer drivers on your PC.
· Download the correct Vista driver for your model.
· Install the driver on your PC.
Important - Do not reconnect the USB cable until the install software prompts the question of how the printer is connected."
peccavi, March 2009