[HELP] Error cannot load (or bundle) 'pg' gem on local windows 7 Ruby on Rails

I posted a question on SO, but no one answered so far.

I just started to learn ruby on rails. I have pg gem installed. And 'pg' is in both gemfile.lock and gemfile. However, it keep getting error, "gem is not loaded, Add gem 'pg' to your Gemfile."

I also have tried follow someone's advice, delete the gemfile.lock and re-bundle. same error came out.

Please look at the link above for details.

Thanks in advanced.