There are many threads and instructions on how to do it on a Mac, it is super easy to do in a Mac, just run the application, and the application would actually guide you how to do it for you
I have been getting this for hours.. updated from 2.2.1 to 3.0 on a 2g iphone on windows and it's showing me the emergency screen , can't even get it to downgrade or do anything for that matter.. any advice please?