Use either string.StartsWith() or string.SubString()
VB Code:
  1. string s = "http://w3c.org";

  2.  
  3. //method 1

  4. if(s.StartsWith("http://"))

  5.     MessageBox.Show("URI has http://");

  6. else

  7.     MessageBox.Show("URI don't have http://");

  8.  
  9. //method 2

  10. s = "http//www.google.org";

  11. if(s.Substring(0,7)=="http://")

  12.     MessageBox.Show("URI has http://");

  13. else

  14.     MessageBox.Show("URI don't have http://");