If you want to change which application opens the .jpg and .jpeg file extensions, then try the following:

Open up the file explorer and go to Tools>Folder Options...
Select the File Types tab and scroll down to the .jpg file extension
Select the file extension then click on the Advanced button
A window should popup with different actions for that file type listed. We're concerned with the "Open" action.
Select the Open action and click the Edit button.
Yet another window will pop up with a filed labeled "Application used to perform action" wich should include the complete path to the application you want to open the file type plus a %1 at the end. Here is an example of what it says on my XP machine:

"C:\Program Files\Common Files\Microsoft Shared\PhotoEd\PHOTOED.EXE" "%1"

The %1 is very important. So, change that path to the path of the default photo viewer for your system and it should work.

- trs