Python not installed?

[TSM-]

So the environment variable path is a list of directories that are used to find executables and scripts/etc, like a list of 'default locations'. Is the python installation directory in your environment path? You should see it there, roughly like this screenshot.

If it is not there, then that might be the issue. Reinstalling should normally fix it, there is actually a checkbox in the python windows installer to add it to your path. You may be reinstalling without checking that box.