I was just installing particle-cli on a new machine and got this error:
Error: Can't find Python executable "python", you can set the PYTHON env variable.
Super obvious what is going on – you need to install python or if you have installed you need to set the env variable.
I installed Python 2.7.10 and selected the following option.
and that will put Python in your path
Launch a new cmd prompt and you should be all set.
Hope this helps.