SSRS 2008 no longer needs IIS to work. This error seems to come directly from SSRS, not from the web server. You need to provide access to your account (MYBOX \ xxx) using the Report Manager interface. (SSMS can no longer be used to grant rights to the report manager).
You will need local administrator access to the machine on which SSRS is installed, since it automatically has system-wide administrator rights in Report Manager. Then just go to http://localhost/reports and give \ MYBOX \ xxx all the necessary security permissions.
MSDN tutorial on user rights management in SSRS.
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