Cannot login to Oracle11g with System username
You mentioned you were on a Windows machine and a brand new DB.  A problem I've run into is the ORA_DBA group.  Windows has a habit of not allowing you to log into an Oracle DB with high level credentials if your OS account is not in the ORA_DBA group.  We found this problem when we migrated to Windows 2008.  I have no experience with Windows 7, but feel like its a safe assumption that it has a security setup similar to 2008, and this may be your problem.