Open up IIS manager.
Right click on the web root in the directory list and left click Properties.
On the Directory tab of the Properties pop-up, click the Create button about half way down on the right side of the page.
Try restarting IIS.
Forgive me if you've done this already, but sometimes it's all it needs.
If your site has any subdirectories with web.config files in them make sure they have no reference to authentication or session states. If they do, you have to use the same process as above to make the subdirectory into an application or point the main web root web.config file to the location of the authentication or session state instructions.
Try hitting that 'Create' button. If I had a pound for every time I forgot to do that before trying to browse my .NET sites...