use sql server for performance and speed
have one table that contains user names, passwords, emails, post counts, so on and so fourth... have na incrimenting feild so each user has their own id
have another table with thread names, number of posts, last poster etc, forum association, thread ids
have a table for the differnet forums, last poster, threads posts etc, number for each forum
Its not too extremely difficult, its just a ton of code. You can find some premade solutions though. www.asp.net has an official forum on it which you can download, but it runs from a dll, so its not open source, there is something called pop forums, and there are several other good ones on it.