How to get programs to open normally again

Alright well my friend we’ll call him the n00b downloaded an ISO from here. The idiot tried to open it by double clicking on it and the “Which program do you want to open this with” came up. He chose Windows Media Player and now every program he uses opens with WMP. The program doesn’t open up, just Windows Media Player. This is all I know because that’s all he told me he did. So just to recap.
n00b Downloaded ISO. n00b tried to open it with Windows Media Player. Now all of his programs open WMP instead of the actual program. Any solutions to try to get programs to open normally again? He’s running Windows 7 Ultimate 64-bit.
Answer #1
Try and open a program by selecting the right app on “Which program do you want to open this with”.