Windows 7: jpg vs JPG

Over night my computer will no longer allow me to open JPG files. However, it still allows me to open jpg files. I can't imagine there is a difference between the two file types, but my computer seems to think so. I have attempted to associate all jpg files by using default programs, but there is not an option for files that use capital JPGs. Can anyone please help?

File extensions are NOT case sensitive.
Meaning : capital letters or lower case letters mean nothing to the system.

Suggestion only :
First and foremost, run a full virus scan with the security program yiou have installed.
Then........
1. Do a system restore. Pick 2 days ago as your restore point.
System Restore

2. Right click at the JPG file > Open With > select the program that is supposed to open all jpg files.

3. You might want to do a system file check, in case you have corrupt files.

SFC /SCANNOW Command - System File Checker

AVG was just run last night.

I know it sounds dumb, but I have checked about 10 of the jpg files and they all opened fine. I also tried about 10 of the JPG and none would open. Certain people are sending the files with caps and others come through in small case. It makes no sense to me.

I am reluctant to run a system restore because every time I have in the past it has screwed up the 300 emails a day I receive.

I have also right clicked the file and it only provides the option of open, not open with.

Followed you guidance for for corrupt files and there are none.

Any more ideas? I am still absolutely dumbfounded how the case of a file extension could create a problem.

My fault. I should have made the provision that you don't have Open With option.
Here is the tutorial to enable Open With into your right click context menu :
Open With Context Menu Item - Add or Remove

Followed your instructions twice, rebooted machine and still no open with. Any ideas?

The message on the JPG file that pops up says "cannot create file: photo.JPG. I would cut and paste it here but it will not let highlight the pop up. Also tells me to "right click the folder I want to create the file in, and then click Properties....

None of the above is ever an option. The JPG thing just fails the sanity check.

BTW - If I do a save as I get the same exact message.

just rename the images to end in .jpg & they will open OK .

No, did not work either. Same message as above. Something about properties on the shortcut menu which makes no sense. For some reason I cannot high light the message so you can see it or cut a print screen.

Your photo viewer might have become corrupt. If using the default Windows 7 viewer (Windows Photo Viewer) try running System File Checker. If any damaged/corrupt files are found run the scan 3-4 times.

SFC /SCANNOW Command - System File Checker

If using some other photo viewer, you could uninstall then reinstall fresh copy.

If using Windows Photo Viewer and SFC shows no problems, try another free viewer and see if it will open those jpg/JPG files.

Top 10 Windows Photo Viewer Alternatives for Windows 7

Did you go to Step #2 .....
2. To Add "Open with" Back to the Context Menu
and click the download button > then follow the instructions #3 to #6.