Running PowerPoint Viewer from commandline on Windows
The default location for PowerPoint viewer is:
32 bit Windows 7: C:\Program Files\Microsoft Office\Office12\pptview.exe
64 bit Windows 7: C:\Program Files(x86)\Microsoft Office\Office12\pptview.exe
I would start by checking if those files exists. If not you could check the registry. HKEY_CLASSES_ROOT in the registry holds information on which application should open which file. You should find something like HKEY_CLASSES_ROOT\.pptx\.
If all else fails you could try to do a silent installation of PowerPoint viewer in the background. Download the installation file in the background and run it with PowerPointViewer.exe /quiet /passive /norestart. More info here.