This doesnt sound like the message you get when unsigned drivers are installed (w764bit requires WHQL drivers), so to solve whether its something to do with the downloading have you a previous driver, or one on a CD you could start install to see if you get the same message, cancel the install of course after you find out.
Try starting IE 8 with no Add-ons might solve it straight up - start - all programs - accessories - system tools -IE8 (no add-ons) and then download the drivers.