Se devi rinomirare il file da .txt a .html puoi
codice:
Private Declare Function CopyFile Lib "kernel32" Alias "CopyFileA" (ByVal lpExistingFileName As String, ByVal lpNewFileName As String, ByVal bFailIfExists As Long) As Long
Per caricarlo in una textbox lo leggi riga per riga e salvi nella text box