Sql server Authentication User

I just downloaded and installed Visual Web developer Express 2010 now trying to Create a new app and when try to create a new SQL server express  DB it ask me about using windows auth or SQL server auth,I'd like to choose SQL server Auth but it says auth failed for any username I use, I used "Admin";"Administrator";"admin" and "administrator" with no success, I'm positive about what was the password I set at the  begining for the SQL server express, what I can't remember is the user name that is set by default for SQL, could somebody refreh my mind and  tell me what is the default user name for SQL server express

Thanks in advance for your help

R Walle
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