Scripts and databases. If your site is designed in FrontPage and your server has FrontPage extensions installed you have everything you need - built in to FrontPage already.
FrontPage has its own forum component, and although it isn't as sophisticated as some of the third party offerings it's all you need for a commercial site like yours - and it's a great deal easier to set up and administer. You can customise the forum as much as you like, and with a bit of effort and imagination you'll have something to be proud of - and without any advertising banners.
In FrontPage, just go to File/new/page or web and select web templates. In the list of web templates you'll see 'discussion web wizard'. Select this, and be sure to tick the option box to add the discussion web to your existing web. Then follow the prompts to create the forum. I think you'll be pleased with the results.
Come back to your thread if you need any further help.