IIS is more of a web server than it is a database. I believe phpBB (linked above) will work with several database configurations, including SQL Server (proprietary database for windows, usually used with IIS in most configurations).
Otherwise you can check out www.aspplayground.net. Don't know if they're free or not, but one of the forums I visit quite regularly is run on this and I always thought it was a neat looking forum. But like was stated above, most PHP run forums aren't restricted to MySQL. That just seems to be the DBMS of choice for PHP because PHP has many built in functions to work with a MySQL database, and it's also free and very powerful, so two free products working together really can't be beat.
What DBMS do you have available? Again, IIS is not a database server, it is merely a proprietary Windows web server (for the most part).
James