To use "conda activate" via Windows CMD, not the Anaconda Prompt:
(in response to okorng's question, although using the Anaconda Prompt is the preferred option)
First, we need to add the activate.bat script to your path:
Via CMD:
set PATH=%PATH%;<your_path_to_anaconda_installation>\Scripts
Or via Control Panel, open "User Accounts" and choose "Change my environment variables".
Then calling directly from Windows CMD:
activate <environment_name>
without using the prefix "conda".
(Tested on Windows 7 Enterprise with Anaconda3-5.2.0)