Hello,
This should be your start page for Windows to get a download for the Z640:
http://support.lexmark.com/index?locale=en&userlocale=EN_UK&productCode=LEXMARK_Z640&page=recommendedDownloads&segment=DOWNLOAD#1
Select the matching download for your Operating System: select the correct Download Now on the right, under Learn More.
When the download is blocked and a warning message shows up under the menu bars, then click this message and permit the download.
Best would be to save the execute file first to a folder, "Download_printer Z640" for example, and run (=dounle click) the driver install file later.
Do not connect the printer until installation is ready, or when a message shows up to power up and connect the device to the computer.
When needing more help, just tell at what point the installation fails in this answer.