Macbook white shuts down for no reason

I have a problem with my old macbook (2008 version) I have had my computer for 5 years but now I am experiencing troubles. I will be busy working on my computer and then all of a sudden it shuts down. It shuts down more frequently, but I don't know why. Is there any way to stop it?

Also, this is what is looks like when my computer restarts: