A Windows Authentication dialog box is popping up when trying to view ASPX; it won't accept any credentials and eventually will display a "Not Authorized" error page.

ASP.NET is getting an Access Denied error when trying to use "C:\Windows\Microsoft.NET\Framework\v1.1.4322\Temporary ASP.NET Files\". The identity used by the application pool does not have read/write permissions to that directory.

"Service Unavailable"

Check the System Event Log (run "eventvwr /s"). You should see and Error and maybe a Warning as well from the W3SVC Source at the time you were trying to view the web site/service. One of these will give you a slightly more specific problem which should be resolved in this document.