OK ... we now have it working. *However* it's possible that the installer is aborting after PhysX and not getting as far as the DirectX, XVid (or even Acrobat) installs.
1) After many failed attermpts, I downloaded/installed Phyx (from nVidia) and updated the video drivers while I was at it.
2) Disabled UAC.
3) Ran the installer "as Administrator".
4) Installed to the *default* directory (C:\ .....)
5) The script then found a newer version of Physx but allowed us to resume, whereupon it went on to install DX9 and Xvid Codecs (which didn't show the first time).
It appears that a standard Win7 has DX11 (11????) installed but PTBoats is looking for DX9 (at least to start with). After getting the "install DX9" option, PTB appears to run fine in both DX9 and DX10 mode.