I have node.js installed and running fine on windows 7 computer. I run
> npm install -g express
> npm install -g express-generator
> npm install -g express-generator@3
> express myproject
'express' is not recognized as an internal or external command, operable program or batch file
Check that you have the path to the express folder in the path. remember that you need to reopen cmd to apply the changes in the environment variables as modifying them does not modify the variables of the currently working processes, AFAIK. If you are changing system-wide environment variables, you may need to restart Windows.
You also may need to check if PATHEXT contains .CMD extension.