Below is the code you need. However, it is strongly recommended that you
implement this as a COM Add-In - it will greatly simplify deployment.
Everything you need to know about that is here:
http://www.outlookcode.com/d/comaddins.htm.
Option Explicit
Dim WithEvents myOlInspectors As Outlook.Inspectors
Dim WithEvents myMsg As Outlook.MailItem
Dim WithEvents myForwardedMessage As Outlook.MailItem
Private Sub myForwardedMessage_Close(Cancel As Boolean)
Set myForwardedMessage = Nothing
End Sub
Private Sub myForwardedMessage_Write(Cancel As Boolean)
myForwardedMessage.Attachments.Add "C:\Temp\myFile.txt", olByValue
myForwardedMessage.Body = "Add this text to the body." &
myForwardedMessage.Body
End Sub
Private Sub myMsg_Close(Cancel As Boolean)
Set myMsg = Nothing
End Sub
Private Sub myMsg_Forward(ByVal Forward As Object, Cancel As Boolean)
Set myForwardedMessage = Forward
End Sub
Private Sub myOlInspectors_NewInspector(ByVal Inspector As Inspector)
If Inspector.CurrentItem.Class <> olMail Then Exit Sub
Set myMsg = Inspector.CurrentItem
End Sub
Private Sub Application_Startup()
Set myOlInspectors = Application.Inspectors
End Sub
Private Sub Application_Quit()
Set myOlInspectors = Nothing
End Sub