I get a lot of questions along the lines of "I just installed software X and now file Y only opens in that program. Why won't it open in software Z, like it used to? Help! I hate this!"
Sometimes seemingly mysterious things happen when you install software. One of those mysteries can be changing file associations. Every type of file on your computer is "associated" with a piece of software. For example, generally bitmaps like .JPG and .BMP files are associated with an image-editing program such as Photoshop. So when you double-click a .JPG file in a file listing, Photoshop opens automatically so you can work with the image file.
The problem is that when you install new software, sometimes it changes those associations. Certain products seem to feel a need to take over the world and change things willy nilly all over the place. So for example, you install software with new your digital camera. During the installation, it decides that all .JPG files will now be associated with it and not Photoshop anymore. It's really annoying.
Fortunately, it's easy to change file associations. But first it really helps to show file extensions in Windows Explorer. The way Windows associates files is by looking at the letters after the last dot in the file name. As I've ranted before, by default, Windows turns the display of file extensions OFF. It's dumb. To change file associations, you need to see extensions. So, go into Windows Explorer by choosing Start|All Programs|Accessories|Windows Explorer.
To show file extensions, choose Tools|Folder Options. Under View, remove the checkmark next to Hide extensions for known file types. And yay, now you can see what you're doing!
From there, find a file that has an incorrect association. In this example, you'd look for a JPG picture file. Right-click the file and choose Open With. Now select the program you want Windows to use to open JPG files. Then click the box next to "Always use the selected program to open this kind of file" and click OK.
Now that program will always be used to open that kind of file. You can easily repeat the process if you change your mind later. For example, I changed my .GIF and .JPG file associations, so that when I double click them, Internet Explorer opens, not Photoshop. Because Photoshop is huge, it takes a long time to open. Sometimes I just want to take a quick look at a photo, not edit it, so I changed the association.