Beh ... cosi' come l'hai detta, puoi scrivere

codice:
    Dim bFirst As Boolean = False

    Private Sub WebBrowser1_Navigated(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserNavigatedEventArgs) Handles WebBrowser1.Navigated
        If bFirst Then
            Button1.Enabled = True
        Else
            bFirst = True
        End If
    End Sub