After the installation of TFS 2010, I was looking through all the parts to check if the installation was successful.
When I browsed to the report manager website, I saw something strange: only the header was visible. As I had this problem before I knew it had something to do with the security settings. So I opened up SQL Server Management Studio, connected to the reporting services server and checked the security settings. But everything seemed OK.
After some research I found out that even if you are logged on with administrative credentials, you might have trouble accessing Report Manager or the http://localhost/Reports site on a computer that is running Windows Server 2008 or Windows Vista. You might need to add those sites as Trusted Sites in Internet Explorer or start Internet Explorer as an administrator. To start Internet Explorer as an administrator, click Start, click All Programs, right-click Internet Explorer, and then click Run as administrator.
More info here.