For specific directory you can use what Yogi suggested
For restriction of navigation, I would suggest you Set the InitialDirectory to your path. If a user selects a another path outside of your path, use the FileOk event to check this and bring the user back to the InitialDirectory
openFileDialog1.InitialDirectory = Path.Combine(Path.GetDirectoryName(Application.StartupPath), "FolderName");
if (openFileDialog1.ShowDialog() == DialogResult.OK)
{
while(Path.GetDirectoryName(openFileDialog1.FileName) != Path.Combine(Path.GetDirectoryName(Application.StartupPath), "FolderName")){
MessageBox.Show("Please use the default folder", "Wrong folder", MessageBoxButtons.OK, MessageBoxIcon.Information);
openFileDialog1.ShowDialog();
}
}
hope this helps