indexof has an overload where you can specify the startIndex:

vb Code:
  1. Dim teststr As String = "testtesttest"
  2. Dim startAt As Integer = 0
  3. Do While teststr.IndexOf("test", startAt) <> -1
  4.     MsgBox(teststr.IndexOf("test", startAt))
  5.     startAt = teststr.IndexOf("test", startAt) + 4
  6. Loop