Connection denied. Please help

Hello, everyone. I'm new in Reporting Services 2005. I encountered a confused connection problem. I'm running a named instance (called 'sql05') of SQL Server 2005 Standard Edition(a SQL Server 2000 instance as my default) on my Windows XP Professional machine. I log in Windows XP as a member of Administrators group. When I came into SQL Server Management Studio and connect to my local reporting server using Windows Authentication, the report server denied my connection request 'actively'. The service is running but it denied my connection. But when I use the same method to connect to my database engine and Analysis Services in Management Studio, no problem has happened. Does anyone have similar experiences? I appreciate for any help. Thank you!Ha, I found the solution. I mistakely altered the settings in Reporting Services Configuation Manager. On the 'Database Setup' page, I mistakely changed the 'Credential Type' from 'Service Credentials' to 'Windows Credentials' and input my WindowsXP username and password. This seems error-prone. When I changes back to 'Service Credentials' and apply it, the problem above solved. Can you help me distinguish between these Credential Types? Which type is suitable?