Why does my computer freeze from strange things?

I run on a very expensive computer and it's great, but a few things make it freeze for no apparent reason. When I play Skype Checkers with my friend, he'll always shake my screen when he's about to lose, causing my computer to freeze. And recently, I tried to convert my iTunes songs to MP3 AND WAV and they both caused my computer to freeze in seconds. Why is it doing this?? It's very frustrating.