What is the default username and password of my SQL Server Express?

It installed automatically with Visual Studio 2010 Ultimate. I didn't create any users for the server or ANYTHING.

I'm using:

string connectionString = @"Server=.\SQLEXPRESS;Database=SportsStore;Trusted_Connection=yes;";

I get error that authentication failed. So the server is being found, but my credentials are wrong. What would the default login and password be?

Edit: Still not working! :(

Here's my connection string:

string connectionString = @"Server=.\SQLEXPRESS;Database=SportsStore;Integrated Security=SSPI;";

And the error message:

Cannot open database "SportsStore" requested by the login. The login failed. Login failed for user 'ToshibaLaptop\Sergio'.