User's authority to admin

I just installed WP3.1 using the Cpanel Packaged Module on the webhosting
server. I set the different name as the admin and different table prefix. My wordpress was installed on http://domain.com/domainblog. It seemed everything okay but the system told me no authority to admin the admin panel after my logon. It still failed even I logon the Control Panel of the hosting to enter the wordpress admin. I don’t know what the problem is. Please help me out?