No,I actually renamed the default instance.... It was SQLExpress or something like that and I changed the name to MSSQLSERVER..
SQL Express defaults to installing as a named instance, and if you changed that to MSSQLSERVER, then it's installed itself as a named instance with that name. If you wanted to install it as a default instance (e.g. not requiring the instance name to be specified when connecting to it) then you should have changed that option in the installer.