A standard method of finding the error is to place msgboxes between the lines of code, something like this:
Set objPPT = CreateObject("PowerPoint.Application")
objPPT.Visible = True
When you run the macro, if you see msgbox "1" and not msgbox "2", you know the problem is with the CreateObject line.
If you see msgbox "2" and not msgbox "3", you know the problem is with the .Visible line.
Since this code is not Qlikview specific, you can create a .vbs file and run it outside of QlikView. I presume it will fail in the same manner. If the problem is not in QlikView, then you start looking at your installation of Microsoft Office.