I just spent a good chunk of time fighting a problem with my SQL Server 2005 Reporting Services installation. I had not previously used it on this machine so it should've been exactly as the SQL Server (and SP2) installation program configured it.
I started by deploying a new set of reports from Visual Studio over to the server, which worked straight away. I then went to the Web-based Report manager and could see my reports deployed, but as soon as I tried actually rendering one of the reports, I'd get an error like this one:
Remote server returned an error: (403) Forbidden
Long story short, the problem was in how the /Reports application references the services in the /ReportServer application, and I fixed it by going to IIS and modifying the /ReportServer application configuration to add "ReportService2005.asmx" as a possible default content page under the "Documents" tab of the application's property page.