Reporting services url's keep asking for authentication SSRS 2008 R2

Hi all.

Just setup SSRS 2008 R2 on our server, however when I try to browse to the /Reports /ReportManager url's I am asked to authenticate. Even if I try entering the local administrator account this is not accepted. Has anyone else had this problem or is there something I have missed in the setup. Note that I have this setup on a specific host header in the setup e.g. ssrs.mydomain.com/Reports

Thanks

P