Thanks for the replies Edmund.
I've finally found out what went wrong. The 64-bit driver for the Asus graphics card isn't or wasn't downloading properly. Despite the 80+Mb file appearing in downloads it wasn't unzipping everything, i.e., it was corrupted in some way. Therefore it wasn't installing the driver hence the problems I had. This happened on two different internet connections so can only assume the problem was with Asus or a remote connection (or something).
Anyway, all sorted now.