I am missing an Authentication Provider for my WSS3 Web application, and
getting a 403 error when I try to connect.

The website was built on another machine, upgraded to WSS3 from WSS2. I
used the Backup utility in Central Administration to back up the Sharepoint
site. When restoring, I am told that the operation was successful, but I
cannot access the website. That is when I found the Authentication Provider
error. This does not exist for other websites (provider is Windows
(Default)). I checked in IIS, on the ASPNET tab, an the provider is set to
Windows as well. Any ideas?

Thanks,

David