Add an environment variable called NODE_PATH and set it to %USERPROFILE%\Application Data\npm\node_modules (Windows XP), %AppData%\npm\node_modules (Windows 7/8/10), or wherever npm ends up installing the modules on your Windows flavor. To be done with it once and for all, add this as a System variable in the Advanced tab of the System Properties dialog (run control.exe sysdm.cpl,System,3).
Quick solution in Windows 7+ is to just run:
rem for future
setx NODE_PATH %AppData%\npm\node_modules
rem for current session
It's worth to mention that NODE_PATH is only used when importing modules in Node apps. When you want to use globally installed modules' binaries in your CLI you need to add it also to your PATH, but without node_modules part (for example %AppData%\npm in Windows 7/8/10).
Hope it works!!