Just want to add something to no1's post:

As far as SubStation Alpha goes. It's a deserted project and just the script type still stands. Don't try searching for the original substation alpha it will take you ages. You might want to try and find alternatives like Medusa on doom9.org forums and just use their interface to do the whole subtitling. I myself prefer just using notepad. As I haven't really found a GUI that will do what I want.