ASP.NET, referring to a DataSet within script

[fortheloveof]

I am using ASP.NET, and the ASP.NET WebMatrix program, latest version. I am writing a page that is supposed to check the username someone has entered into a textbox against the list of users from the database, so that two people can't have the same username. Simple, right? Not when you're me.

I know what I'd like to do, I just have no idea how to write the following in ASP.NET. This is the pseudo-code for it.
"If txtUserName.Text = Currently Selected Record.Username then"

I can put a <asp:repeater> in the code and use <%#Container.DataItem("Username")%> to print the Username which is great, but that doesn't work within the <script> tags at the top of the page. I just want to know how to refer to the records I've just loaded. What is it called?

Also, what is the difference between a dataSet and a dataReader?

Function GetCustomerList() As System.Data.IDataReader
        Dim connectionString As String = "Provider=Microsoft.Jet.OLEDB.4.0; Ole DB Services=-4; Data Source=" & server.mappath("db\super.mdb")
        Dim dbConnection As System.Data.IDbConnection = New System.Data.OleDb.OleDbConnection(connectionString)
        Dim strUN

        Dim queryString As String = "SELECT Customer.Username FROM Customer"
        Dim dbCommand As System.Data.IDbCommand = New System.Data.OleDb.OleDbCommand
         dbCommand.CommandText = queryString
         dbCommand.Connection = dbConnection

         dbConnection.Open
         Dim dataReader As System.Data.IDataReader = dbCommand.ExecuteReader(System.Data.CommandBehavior.CloseConnection)

         Return dataReader
End Function

Sub cmdSubmit_Click(sender As Object, e As EventArgs)
        GetCustomerList()
        If txtUserName.Text = !!!WHAT! WHAT do I have to do here!!! Then
              lblUsed.Text = "That username is in use. Please choose another."
        else
              lblUsed.Text = "OK"
        end if
End Sub

Many thanks for your help. This is totally frustrating.

[fortheloveof]

...well I solved that one.

The necessary code was simply this:

While dataReader.Read()
                 if dataReader("Username") = txtUsername.Text then
                     label1.Text = "Username is already taken!"
                 end if
             End While

This .Read() fiasco apparently replaced the old .MoveNext from regular ASP. When there are items left, .Read() will return "True". Otherwise, i.e. when there are no more records, it will return "False"... hence the while loop.

This is why I hate computers. ;)