Computer help Windows 7?

Help! I was trying to fix a problem my father in is having with his computer following the instructions that were sent to us through tech assistance with a program he wants. I opened the file it said to and Windows gave me a list of what to open it with. I chose "Windows Media Center and clicked ok. I didn't realize that the box at the bottom that tells the computer to open all programs with that file was checked.

Can someone please tell me how to get everything back like it's supposed to be?