As usual, the internet isn't always 100% up to date.
Looking into the use of npm config, the documentation states that the global npmrc file lives in $PREFIX/npmrc - however, when you actually look for it, you'll find that it actually lives in $PREFIX/etc/npmrc
You can find the value of $PREFIX by running:
npm get config prefix
On Windows, this'll be in your user's AppData folder, normally.
So if you're looking to modify where npm throws cache data, you can run the following:
npm config set cache C:\myfolder\npm-cache --global
As I couldn't find a sample npmrc file online in about a minutes worth of Google, I figured it would be worth posting what the contents look like as well:
Of course, there'll come a time when this post is out of date too - but hopefully that won't be for a while yet!