If you would like to change the default program, that starts when you double click a file, you need to change the file associations.

The first step is to make Windows Explorer show you the extensions of files. The default setting in Windows is not showing the extensions, so you need to change that. Open the Windows explorer and click the "Tools" menu and "Folder options". In the "View" tab band scroll down until you see the "Hide extensions for known file types". This option needs to be unclicked (no tick there). Now, after you click ok, you will be able to see the extensions for all the files.

Now you need to find a file that has the extension that you would like to change the association for. Right click on any file with that extension and choose the "Open with" option. If you see a warning window, then click "open with" again. A window will open with a list of all available programs. Optionally you will see a window saying "Windows cannot open this file". If this happens, click "Select the program from the list" and OK.

Now you will see a list of programs, and you have to choose witch program you would like to use to open files with the selected extension. After you click ok, all the files that have the selected extension will always open with the chosen program.

Be careful while using this procedure, because most of the files have the appropriate default program selected, and if you change it, your system can have serious problems, so use this option only if you are really sure that you do know, what you are doing.