I'll suggest 2 options:
1. Our book,
ASP.NET 2.0 Website Programming Problem Design Solution includes a fully functional website starter kit in for ASP.NET 2.0 in C#. The starter kit site includes a lot of other functionality that you don't mention needing (articles, ecommerce, etc) so it might be overkill. And the book isn't really for beginners. It assumes you know some ASP.NET but just need to learn how to put it all together, use a 3 layer model, etc.
2. If you just want the forum part, check out Yet Another Forum.NET:
http://www.yetanotherforum.net/
It's ASP.NET 2.0 and C#.
Jim Minatel
Acquisitions Director
Wiley Technology Publishing
WROX Press
Blog:
http://wroxblog.typepad.com/
Wrox online library:
http://wrox.books24x7.com