I've had to re-install Opus 8.2.0.2 from scratch (Windows frumps again).
Before if I double clicked (left button) on a picure, movie or sound file, it used to play with IrfanView by default, not the internal Viewer. I can not remember how this was done. Having looked through the Help file, but am unable to find the solution. Anyone know how I did this? Thanks in advance 
.
See the FAQs, specifically the section about the viewer.
I found the solution for this from another source, but thanks to anyone who looked into it.
In case anyone else has this problem, here's a solution:
Opus default file handling
The menu sections for media file handling:
Settings/Preferences - 'Double Click' tab
Uncheck 'Use internal sound player for WAV files'
Uncheck 'Use internal picture viewer for: unregistered file types/all recognised pictures'
Can also effect default actions on other file types with this section of Preferences.