One way:

First set up a signature with the boilerplate text. Then paste this
code in a standard module in the Outlook VBE.

(Note: I did not write this code, it was adapted from outlookcode.com
and rondebruin.nl)

Sub SendBoilerReply()
   Call SendStockMsg("Signature name", "Filename/path")
End Sub

Replace "Signature name" with the name of the signature you created,
and replace "Filename/path" with the fully qualified path and filename
of the file you want to attach. For example, if your signature was
named "Boilerplate Client Response" and you were attaching "C:
\ThankYou.PDF", the code would look like this.

Option Explicit
Sub SendBoilerReply()
   Call SendStockMsg("Boilerplate Client Response", "C:
\ThankYou.PDF")
End Sub

Create as many signatures as you need, just copy and paste the above
code and change the references accordingly. If you attach the macro to
a toolbar button, it will be even more user friendly (see "How to
assign a macro to a toolbar button" at  http://codeforexcelandoutlook.com/ResendMsg.html
if you need placement assistance).

Then paste in this code in the same module:

Sub SendStockMsg(SigName As String, Optional sFile As String)

Dim myItem As Outlook.MailItem
Dim olNewMailItem As Outlook.MailItem
Dim SigString As String
Dim Signature As String

   ' get valid ref to current item
   On Error Resume Next
   Select Case TypeName(Application.ActiveWindow)
       Case "Explorer"
           Set myItem = ActiveExplorer.Selection.Item(1)
       Case "Inspector"
           Set myItem = ActiveInspector.CurrentItem
       Case Else
   End Select
   On Error GoTo 0

   If myItem Is Nothing Then
       MsgBox "Could not use current item. Please select or open a
single email.", vbInformation
       GoTo ExitProc
   End If

   SigString = "C:\Documents and Settings\" & Environ("username") & _
               "\Application Data\Microsoft\Signatures\" & SigName &
".htm"

   If Dir(SigString) <> "" Then
       Signature = GetBoiler(SigString)
   Else
       Signature = ""
   End If

   Set olNewMailItem = myItem.Reply

   olNewMailItem.HTMLBody = Signature & olNewMailItem.HTMLBody

   If sFile <> "" Then
       olNewMailItem.Attachments.Add(sFile)
   End If

   olNewMailItem.Display

   myItem.UnRead = False

ExitProc:

Set olNewMailItem = Nothing

End Sub

Function GetBoiler(ByVal sFile As String) As String
' from Dick Kusleika
   Dim fso As Object
   Dim ts As Object
   Set fso = CreateObject("Scripting.FileSystemObject")
   Set ts = fso.GetFile(sFile).OpenAsTextStream(1, -2)
   GetBoiler = ts.ReadAll
   ts.Close
End Function

The filename argument is optional, so if you had a stock reply you
wanted to send, but didn't want to include an attachment, this code
would work:

Sub SendBoilerReply()
   Call SendStockMsg("Signature name")
End Sub

This code will work if you select one item when viewing your mailbox,
or on the currently open mail item. Note that it looks for the
signature file in this folder:

C:\Documents and Settings\"username"\Application Data\Microsoft
\Signatures\

If your folder is different, change the path accordingly. (search for
"signature name".htm to find the correct folder)

ps- your stock reply sounds a bit like the "bug letter" urban legend

http://www.snopes.com/business/consumer/bedbug.asp

HTH,
JP