powerpoint viewer

I have windows xp which I had to do a restore on a couple of months ago. Recently I was trying to open a file that I have had quite a while that was a presentation on powerpoint viewer. I had saved it before the restore on a disc. Now I can't play anything with powerpoint. I even tried downloading it again and when I open those files it tells me that I can only open this file with an already downloaded program. How do I get powerpoint to work again so I can use those files.