Windows will usually open a specific file type in the same program, known as the default program, all the time. For example, all .doc files are usually opened using Microsoft Office while JPEGs can be viewed using Windows Picture Viewer. However, sometimes when you install other software on your machine, it can overwrite the default and you'll find files being opened up in other programs.
This can easily be rectified though. Here's how to change the default program a specific file type is opened with in Windows 7.
From the Start menu select the Control Panel.
Open the Program section and then click on Make a file type always open in a specific program from the section labelled Default Programs.
Windows will now display a list of all the various file extensions and the default program currently used to open it. To change the default program, simply click on the extension your require. For example .doc. Now press Change Program in the top right-hand corner of the window.
You'll now be offered a list of programs installed on the PC that can open this file type. Select the one you want and press OK. The default program will now be changed.
Alternatively, you can right-click on any file with this extension and select Open with from the drop down menu. Press Choose default program from the menu and you'll be presented with the compatible programs installed on the machine. Select the one you want and then make sure the box next to Always use the selected program to open this type of file is checked. Press OK.