337.91? That only seems to be offered by nVidia for the new Titan Z. If you put in any other graphics card (for example your 770) into the options on the nVidia site, the newest on offer is the 337.88, so I recommend you try that one, as opposed to the 337.91.
It is a bit strange, isn't it? But I think I remember them doing it before with a new release; releasing a driver that only works with the specific card. Either way, give the 337.88 driver a go; it should hopefully resolve the problem of it not detecting your 770.
EDIT: just double checked, and yep, the nVidia site lists the Titan Z as the only supported product for the 337.91 driver.