how to change reporting service site seurity setting?

I have installed SQL Server 2008 with reporting service on a named instance MSSQLSERVER2008. The Service accouint  for reporting service is Network Service.
then I tried to access Report manager with following url:
http://localhost:8088/Reports_MSSQLSERVER2008

In filefox, it will always ask me user and and password in a pop-up windows.
in IE, it give me error message:
The report server cannot decrypt the symmetric key that is used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content. (rsReportServerDisabled) 

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Bad Data. (Exception from HRESULT: 0x80090005)

But I did not make any change. Everything is set as default value.

How to resolve this problem?