till some days ago when I tried to open a jpg file it opened in Irfanview program.
Lately the files open in Windows viewer.
I would like to se the files in Irfanview. What should be done to return to the former configuration
Please advise
Thanks

See IrfanView and check the FAQs page for making Irfanview the default viewer.

Right click any .jpg file. Choose Open With/Choose Program. Click on IrfanView, then below the "window of programs" checkmark "Always use the selected program.......".