Both machines (ProDesk 400 G2) with this issue have had full updates, had the 2013 and 2013 (x64) Redistributables installed. The KB2999226 update specifically installed.
Have run the DOS sfc /scannow - reports that all is good and nothing is missing. Every possible (read sensible) fix as suggested on the net and from the HP forums have been tried to no avail.
One machine will work with Office currently only because I did a complete reset of the OS - turned off ALL updates at time of install AND any future updates and it works. Each time updates are installed the problem comes back.
Taking ours out of the productivity of the machine each time we need to do this.