I had the same issue, trying to install bower with npm install -g bower
I think this was because node was installed by another user, not me.
I uninstalled node, and then I reinstalled it. During installation, I saw this text for the option Add to PATH > npm modules:
Message in node installation
After node installation, I executed npm install -g bower
again. And now bower works.
Sure is not necessary reinstall node with own user, like me. Solution must be via NODE_PATH or PATH variables, as other users have explained.
This is only to remark that this problem occurs only if node has been installed by another user (or if during installation the option Add to PATH > npm modules has not been marked).