Hello Fircxo, and welcome to Seven Forums.
You should be able to right click on a .ppt and .pptx file, click on Open with, select Choose default program
, then select Microsoft Powerpoint. The tutorial below will help show you how to, but you do need to already have a program like Microsoft Office or Powerpoint installed to be able to open .ppt and .pptx files with. Open With - Change Default Program
Hope this helps,