Try the CellEnter event:
void dataGridView_CellEnter(object sender, DataGridViewCellEventArgs e)
{
if (((DataGridView)sender)[e.ColumnIndex, e.RowIndex].Value != System.DBNull.Value)
Textbox.Text = ((DataGridView)sender)[e.ColumnIndex, e.RowIndex].Value.ToString();
}
Kjetil