Well.

I can offer a few suggestions.

phpBB is very good stuff - free, pretty easy to use, and great to learn on if you're interested in learning PHP.  It's pretty forgiving on webservers, so if you're on a shared webhosting plan, that will help.

I personally use Invision.  It's sort of free, sort of not.  You can download it and use it, but you can't take out their tags.  They are very small though  - so you don't notice them.  If you pay for it, you can get support and what not.  However, in terms of performance and features, there is no difference between the free version and the paid version.

Both use the database software MySQL.  That's also free.  

The webserver needs to be running Apache 1.x (2.x is not recommended for production) or Windows IIS5 or IIS6.  You will also need to have PHP version 4.x regardless of what platform you're running on.  If I had choice, I would take the Apache on *nix over IIS.  For my site, I use Solaris for the database server and FreeBSD and Solaris for the front.  

If you've got more questions, let me know.