Please think carefully before calling anything "Adobe" - this was at the root of your problems in the first place!

Many thanks, ~graffiti (and Test Screen Name), for your help. You have fixed the problem.  All jpg files now open using the Windows format in the 'Open with' option box and have all been converted to this 'default program'. Apologies for being slow on the uptake with this stuff!

I'm still not clear as to how the Canon installation changed the 'Adobe' set-up with these files, which is what appears to have happened, but never mind...it's sorted now, thanks to you!

As a rider to this, just thinking about Adobe and PDFs, if I ever wished to convert a jpg from Windows to a PDF in Adobe, could this be easily done?

Here is the key question. Until you answer it you can’t fix The problem. You say you used to save JPG files. What app did you use? If you really can’t remember, please describe the sort of thing you use to do.

Thank you.

I am certainly not suggesting that Adobe's software is not working as intended or that anyone needs to look into anything... I'm only seeking to understand why clicking on an Adobe file saved in jpg format a month, a year, five years ago now doesn't work any longer.  I have never knowingly used any other app (although I can see that the 'rogue' files will open in WMP) : they have always opened immediately, apparently as Adobe files (although I believe you when you tell me they are not!)

You are unquestionably correct that the pop-up box, which only started appearing since this became a problem, indicates there is a 'mistake' : what I'd like to understand is where the problem lies and how to correct it. 

Clearly something has changed and I will have to, as you advised initially, figure out what it is.  I have never used Photoshop, however that suggestion lit up the thought that it may be something connected with a new Canon printer, installed recently, and coinciding with this issue, which has a piece of unwanted Canon software which has kept popping up, PictureGarden or something... could that have an influence or am I barking up the wrong tree?!

And yes we recognise the problem completely. There is no mystery here.

Please stop thinking "Adobe files". So long as you think you have "Adobe files" and an app called "Adobe" you won't be able to solve this. Know what apps you have, which apps you use and that none of them are called just "Adobe".

So, what is the other app you were using - as well as Acrobat Reader? To go forward you must find out this other app. Go through the START menu list carefully.

Thanks for sticking with this query - I appreciate your help.

I don't understand what is causing the problem...  I did not have to use any other application or software (as far as I'm aware! Was it a default item associated with these files for years, running in the background?) to open these jpgs, stored, as you say on the "system" as Adobe files. They were saved as Adobe files and always, until recently, opened instantly they were clicked, as Adobe files : just as they would with pdfs - absolutely no difference in use.  Indeed it wasn't until I checked Properties that I learned there was a difference!

So where is the gremlin which has crept in to prevent these files from opening?!

A mystery, it seems... I was hoping someone at Adobe might recognise the issue and be able to help.

This is confusing. Windows says this is an Adobe Acrobat file BECAUSE A MISTAKE WAS MADE which set Acrobat Reader to open JPEG files. It doesn't make them actually Acrobat files, any more than sticking a sign on an orange which says "apple" makes it so. You need to undo the mistake.

Let's step back. You wrote "files previously saved as jpgs in Adobe for years ". Now, we now for sure that's a different app because we know that Acrobat Reader can't save JPGs. So what app is that? A possibility is Photoshop, or Photoshop Elements.

Thanks for your prompt response, ~graffiti,  however I am still baffled.  The only version of Acrobat used by my PC for years has been the free download, Adobe Acrobat Reader DC. It has successfully saved and opened files such as the example below (of which I have scores if not hundreds) : you see from Properties, below, that this (and most of my jpg files) is an Adobe file. In the last week or so the facility to open these Adobe jpgs has suddenly disappeared. The cause, for all I know, may not be directly connected to Adobe, however it would be strange if Adobe have not picked up on this issue and be in a position to provide information and support.

If you still find this puzzling please go back over your original question and replace every word “Adobe” with the name of the actual app you mean. I think there are at least two different apps in your life, maybe three.