プログラムが"C:\Program Files\MyProg"にあり、
"C:\Program Files\MyProg\file\index.txt"を開く場合

Dim strBuff As String
Dim intFileNum As Integer
Dim strFullPath As String 'フルパス
Dim strSoutaiPath As String '相対パス

strSoutaiPath = "\file\index.txt"
strFullPath = App.Path & strSoutaiPath

intFileNum = FreeFile()

Open strFullPath For Input as #intFileNum
Line Input #intFileNum, strBuff
Close #intFileNum

Label3.Caption = strBuff