Do you have something called IPhotoExpress made by a company called Ulead installed on the PC? If so, reinstall it and that should fix the problem.
Or you should be able to just use InternetExplorer to view .jpg files. Try this and see if it fixes it. First be sure you don't have any instances of Internet Explorer open. Then click on the folllowing,
Start>Settings>ControlPanel>InternetOptions.
Now click on the "Programs" tab. Now at the bottom, select "Internet Explorer should check to see if it's the default..." and click on Apply and then OK.
Now start IE and when prompted, click on Yes to make IE your default. That should reset the file associations.
Now try to open a .jpg
Let us know what happens. If that doesn't work, there are other ways to reassociate a file manually.