I have been look around for Free/Open Source ASP.NET CMS / Portal systems for a while now, and have seived it down to two different ones.
Umbraco - http://umbraco.org
I actually went for Umbraco in the end and would never look back, its incredibly easy to install and use
To install you can use the web platform installer to install it and the AMAZING amount of free projects you can EASILY install with a couple of clicks make it by far the best CMS out there
http://our.umbraco.org/projects
If you are unsure where to start have a read of this
http://www.blogfodder.co.uk/post/A-Complete-Newbies-Guide-To-Umbraco-CMS.aspx