|i also think SQL Server Express is a great option. it's free and has the GUI you desire. you can also query the database from other software (i.e. a C# program) if you wanted to, assuming you know how to build that.|
another, very similar option would be SQLite, which i definitely recommend. it is truly "standalone" in the sense that there is no SQL server that needs to be running and active as there is with SQL Server. the database is stored as a file on your computer and the database can be encrypted using SQLite's built in features if you want.
again, you can write your own code to control the database...OR you could also use any one of the MANY graphical user interfaces that are available as a frontend for SQLite.
SQLite, like SQL Server Express is also free.