You probably can't find it because Windows Photo Viewer is not implemented as an EXE file. Rather, it is a DLL file.
The actual file is named PhotoViewer.dll and is (or should be) located in C:\Program Files\Windows Photo Viewer (and, if you have a 64-bit version of Windows, also in C:\Program Files (x86)\Windows Photo Viewer.
How are you trying to use Windows Photo Viewer, and what happens when you try?
If you right-click on an image file (e.g., *.jpg, *.bmp, *.png, etc.) and select "Open with > Choose default program" is Windows Photo Viewer included in the list?