I want to embark on learning a totally new language for web back-end development and I've narrowed down my choice between these three:

GWT 
Ruby
Python

Do you know or recommend of any sites that walks you through building a simple site using these technologies that could be easily deployed and tested?
By the way, I am running a Windows OS, so please let me know if there is anything I may need to configure on my machine to start learning these tools.
On the front end side of development, I would also like to see some samples that greatly exemplifies being able to use the technologies in its full potentials.
Totally appreciate your suggestions and responses. Thank you.
Angelo
I would personally go with Python/Django (you can get nice overview of the technology on djangobook.com) but Ruby/Rails is probably equally good.
And about GWT ... hmm we are about to start GWT project at work and I am really scared. Code looks terrible bloated , generated HTML is huge , performance can be a problem and no SEO.
GWT or Python or Ruby for Web Development