So, another strange problem when installing Windows 7 on my partner’s laptop. Every time I downloaded the nVidia drivers, either from the official site or from laptopvideo2go.com, unpacking them yielded an error saying either
Non 7-zip archive. or
7-Zip: Data error.
I tried unpacking the archives manually with 7-Zip, and each time a different set of files would fail CRC inside the archive! So, something is probably going wrong with nVidia’s servers, since other things seemed to be downloading fine.
Forum searching yielded that people just kept retrying the download and eventually it worked. I did this, to no avail. Then I shamefully caved in to superstition and tried downloading with IE (where I had previously used Firefox), and it then worked.
I'm not sure whether I just got lucky with the continual re-downloading, or whether using IE had something to do with it.