La cuenta sa esta deshabilitada por default
para habilitarla...
Para habilitar el SQL Authentication :
Código:
Enabling SQL Authentication
STEP1:
You need to enable SQL Authentication on the machine. This is complex in SQLExpress and you need to edit the registry. Copy the following lines of text onto a Notepad and save it with extention .REG.
-------------COPY BELOW---------------------------------
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.1\MSSQLServer]
"LoginMode"=dword:00000002
-------------COPY TILL THIS---------------------------------
STEP2: Double Click on the .REG file to change the registry. This change the Authentication Mode to enable SQL Authentication.
STEP3: Restart the SQL Server Express Service or reboot the machine for registry change to take effect.
y por si acaso, para habilitar el modo de conexion remoto....
Configuring SQL Server Express 2005 for Remote Access 
Saludos