To install npm on windows just unzip the npm archive where node is. See the docs for more detail.

npm is shipped with node, that is how you should install it. nvm is only for changing node versions and does not install npm. A cleaner way to use npm and nvm is to first install node as it is (with npm), then install the nvm package by npm install nvm

Edit: It seems like new installers do not have this problem anymore, see this answer by Parag Meshram as my answer is likely obsolete now.

Original answer:

Follow these steps, closely:

• http://nodejs.org/download/ download the 64 bits version, 32 is for hipsters
• Install it anywhere you want, by default: C:\Program Files\nodejs
• Control Panel -> System -> Advanced system settings -> Environment Variables
• Select PATH and choose to edit it.

If the PATH variable is empty, change it to this: C:\Users\{YOUR USERNAME HERE}\AppData\Roaming\npm;C:\Program Files\nodejs

If the PATH variable already contains C:\Users\{YOUR USERNAME HERE}\AppData\Roaming\npm, append the following right after: ;C:\Program Files\nodejs

If the PATH variable contains information, but nothing regarding npm, append this to the end of the PATH: ;C:\Users\{YOUR USERNAME HERE}\AppData\Roaming\npm;C:\Program Files\nodejs

Now that the PATH variable is set correctly, you will still encounter errors. Manually go into the AppData directory and you will find that there is no npm directory inside Roaming. Manually create this directory.

Re-start the command prompt and npm will now work.