I googled "asp.net forum software" and got some decent-looking hits, like Instant Forum.net and ITCN Asp.Net Forum, and dotNetBB. I haven't worked with any of those packages, but I presume it's important the package work with the .net platform.
I've been looking as ASPPlayground myself. It's used to power the MS Frontpage community at Outfront, which is an active community that a number of WW members (and some mods) also participate in. Thomas Brunt, the Big Kahuna at Outfront, is most helpful and would likely answer your questions.