After installing angular-seed project, i did the following steps:
Cloned the repository :
git clone https://github.com/angular/angular-seed.git <
If the accepted solution did not work for you. Try the following. This solved the issue in my system.
Make sure the locations of both npm and http-server are added in environment variable 'PATH' (Both user variable and System variable).
If it is not present,
add C:\Users\username\AppData\Roaming\npm
and C:\Users\username\AppData\Roaming\npm\node_modules\http-server
to environment variable 'PATH'