Well, generaly, you want to go to Folder Options in the Control Panel. Once there, you can click on the file-types tab and choose the file extension. From there, you can edit the functions, such as open. You can alter what program it uses to conduct that function.
A sometimes simpler way is to RIGHT CLICK on the file icon then choose OPEN WITH, pick the program you want then check the box that says ALWAYS OPEN WITH THIS PROGRAM.
