installing libxml-ruby under windows

Hi

I´ve tried to many times on differnet ways to install libxml-ruby under
windows.

What I tried

I´m using instantrails as enviroment.... ok

after "gem install libxml-ruby" my rails-app gave me the error

no such file to load -- xml/libxml

what´s the reason?!?

What i have to install else?!?

thanks for help!