System.IO.StreamReader sr = new System.IO.StreamReader(@"d:\balamurali\bb.doc");
string content = sr.ReadLine();
int lineno = 1;
while (content != null)
{
    if (content.IndexOf("SomeText") > 0)
            MessageBox.Show("The specified text found in line no: " + lineno.ToString());
    lineno++;
    content = sr.ReadLine();
}