the basic funtion of a forum is not too difficult. You are basically authenticating users, then allowing them to provide input which you store in a database. You display the contents in your forum based on categories, sub-categories, etc...
What gets challanging is to optimize the forum, add additional features, stats, messaging, etc.. its takes quite a bit of work to complete this type of project.
if you are looking for an asp.net forum, try jitbit.com. They will provide you with a free copy of an unlimited version of their asp.net forum software. The only catch is that the code behind is complied, so you only have access to the html files. If you want the code behind, you can purchase it for a few hundred dollars.
What I like about their concept is that they use a master page so making changes to the forum site is easy... so if you want it to match your website look and feel, you can modify this master page file by adding a header and footer and also modify the CSS (style sheet) as well.
i have a copy of that forum software running, feel free to take a look. You'll notice the nice integration with the look and feel of the main site.