To solve your problem please follow this steps:
1. Firstly navigate to "Control Panel" then "System"
2. Click on "Advanced system settings" on the left
3. Click on "Environment Variables"
4. Then search and click on your "Path" variable
5. Then click "Edit"
6. Then you must add "C:\"to the environment variables field, if you are using Windows7, then
separate it by a semicolon from the existing entry. If you are using Windows10, just simply click
"New" to add.
7. Reopen the Command Prompt and try to add your new path
If it is confusing to your or it is still not working let me know in the comments.