как-то так:

object findText = ttt;
int start=Application.Selection.Start;
int en=Application.Selection.End;
            Application.Selection.Find.ClearFormatting();
            if (Application.Selection.Find.Execute(ref findText,
                ref missing, ref missing, ref missing, ref missing, ref missing, ref missing,
                ref missing, ref missing, ref missing, ref missing, ref missing, ref missing,
                ref missing, ref missing))
            {
                MessageBox.Show("Text found.");
            }
            else
            {
                MessageBox.Show("The text could not be located.");
            }
Application.Selection.Start=start;
Application.Selection.End=en;