I Can't Connect To My Local Sql Server Express Db. Please Help

Hello,
I need help to connect to my local sql express db in visual studio 2003 in vb.
It currently works with the production db information and I need to test a few things, So i'd like to point it to the local db.

It works this way:
Const CONN_STRING = "server=johndoe;user id=john;password=doe;database=johnd"
Dim conn As SqlConnection = New SqlConnection(Me.CONN_STRING)
conn.Open()

I want to point it to my local db and it doesn't connect.

        Dim connectionString As String = "Data Source={myusernameSQLEXPRESS};Initial Catalog=johnd;Trusted_Connection=yes"
        Dim conn As SqlConnection = New SqlConnection(connectionString)

        Try
            conn.Open()
        Catch ex As Exception
            Response.Write(ex.ToString())
        End Try

and even that doesn't work.

When i go to my sql server 2005, it says
MYUSERNAMESQLEXPRESS (SQL SERVER 9.0.3042 - theworknetworkmyusername

what am i doing wrong ?
Thanks.