Can I write an npm script that runs slightly different on Windows?

I am using the scripts feature of npm to conveniently start my service by typing npm start. In my package.json file, I have this:

{
    ...
    "scripts": {
        "start": "NODE_PATH=. node index.js",
        ....
    }
}

The NODE_PATH is the troublesome part. I do this so I can write in my javascript files: require('lib/mymodule'); instead of require('../lib/mymodule'); or some variant. There is a different syntax for setting an environment variable for a command on Windows. It would be cmd /C "set NODE_PATH=. && node index.js".

How can I accomplish running the right start command based on the operating system?