Above answer didnt work in win 7 so went to start -> help and typed file types Heres what MS has to say
Change the program that opens a type of file
If you double-click a file in Windows and it opens in the wrong software program, follow these steps to choose the program you would prefer to use. You can change this setting for an individual file, or you change this setting so Windows opens all files of the same type in the software program you choose.
Open the folder that contains the file you want to change.
Right-click the file you want to change and then, depending on the type of file, either click Open with or point to Open with and then click Choose default program.
Click the program you want to use to open the file.
Do one of the following:
If you want all files of that type to open in the same software program, select the Always use the selected program to open this kind of file check box, and then click OK.
If you want only that file to open in the software program this one time, clear the Always use the selected program to open this kind of file check box, and then click OK.