Just spent another half of day solving the same problem already solved before.
If you have these symptoms:
* Http error 503 Server Not Available
* System login dialog even as you have no Windows authentication enabled
First of all, look at MS SQL Server Reporting Services configuration.
It creates TWO sites during install at localhost:80/Reports and localhost:80/ReportServer.
You should move both of them to other ports using Reporting Services Configuration Manager utility
The deep root reason is described here:
Another reason possible is Skype messenger. It also likes to take the port 80 preventing others from using it.
PS: Don 't forget about another very simple reason for Error 503:
You might be running your site in IIS under an Application Pool with user account credentials, and your password has just expired!