Ha... I figured it myself.. Just had to go about it a bit differently.
private void btn4_Click(object sender, RoutedEventArgs e)
{
FolderBrowserDialog folderselect = new FolderBrowserDialog();
if (folderselect.ShowDialog() == System.Windows.Forms.DialogResult.OK)
{
if (System.IO.Path.GetPathRoot(folderselect.SelectedPath) == windir)
{
tb2.Text = "You have selected the system drive for a backup destination. Please select a different drive.";
}
else
{
tb2.Text = "";
}
tb1.Text = (folderselect.SelectedPath);
}
}