Right click the SQL Server service and go to properties. Look at the full path to the executable and see if there is a -s flag at the end followed by some text. If there is, this means that you have a named instance installed. If you DO have an instance installed, then you'll need to include that in the server name when connecting in SSMS. For example, if the instance name is SQL2008 you would connect to the instance by using the following server name: LOCALHOST\SQL2008.