Two cents

Code:
Public Class Form1


    Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) Handles Button1.Click
        Dim stpw As New Stopwatch
        Dim passGen As New RandPassGen
        stpw.Reset()
        stpw.Start()
        For x As Integer = 1 To 10000000 'generate 10,000,000 passwords
            Dim s As String = passGen.NewPass
            'Debug.WriteLine(s)
        Next
        stpw.Stop()
        Label1.Text = stpw.ElapsedMilliseconds.ToString("n0")
    End Sub

    Class RandPassGen
        Private Shared ReadOnly _PRNG As New Random
        'define the characters to be used in the password
        Private Shared ReadOnly _UseChars As New System.Text.StringBuilder("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890")
        Private Shared ReadOnly _MinChars As Integer = 10 'minimum password size
        Private Shared ReadOnly _MaxChars As Integer = 10 'maximum password size
        Private _pass As New System.Text.StringBuilder

        Public Function NewPass() As String
            Dim sz As Integer = _PRNG.Next(_MinChars, _MaxChars)
            Me._pass.Length = 0 'new password
            For idx As Integer = 1 To sz 'generate the proper amount of characters
                Me._pass.Append(_UseChars(_PRNG.Next(0, _UseChars.Length))) 'randomly selected from characters to use
            Next
            Return Me._pass.ToString
        End Function
    End Class
End Class