phpBB2 is your best bet for learning (IMO), it does not need linux, it has the largest database of add-ons and because of its already long lifetime, it has a massive and helpful user-base in its support forums.
It has been losing popularity over the last couple of years but as soon as phpBB3 is finally released it looks like it will take its place as the number one again.
Another reason to use phpBB2 is it will allow you to get to know the community while the wait for phpBB3.
If you haven't got much time, and want to see what the new phpbb3 can offer, just try it out on this host : create forum
Hope this can help!