Mark,
Take a look here at the phpb2 forum. This is the default version.
http://www.phpbb.com/phpBB/It is fairly simple to install and you have the ability to change the look and feel by the way of hundreds of different skins. I use it on my server and it has very little overhead and is written in PHP.
I HIGHLY recomend it.
Make sure and use Dan's how-to, however there is a newer version (2.0.6) that has some security and bug fixes that I recommend. Dan's how-to links to the older (2.0.4) version so make sure you download the newer version. The installation should be the same.
Good Luck,
Paul