If you click Start, then type Default Programs in the search box you will get it. Click on the second option, scroll down to the extension you need. Click on that and click the button Change Program. Click Browse and navigate to the program you want to use, click open, select "Always use the selected program to open this kind of file", click ok and you should be fine. Looks like this is account specific, so you may have this problem with any other accounts you set up on the computer.
