Incrament serial number up or down using txt file and directory search

Hi Everyone,
Any help on this issue would be greatly appreciated, even if it is just to tell me I'm on completely the wrong lines. I have posted on this issue before; http://www.excelforum.com/excel-prog...ml#post3513111

What I am trying to achieve is generating a unique serial number that can not be duplicated.
I want to be able to open a new document from an excel template and the serial number be generated from a text file (got that bit down). Then I want a loop to be run to check the folder the file was saved in to check if a file exists with the same serial number (the file will be saved with the serial number as a name). If a file is found the number stored in the txt file is decrease by -1 then that result is displayed. If no file is found then the result stays the same and that is displayed (when my current loop is run it displays the msg box to say "nothing found" then the serial number disappears).
Sorry if this an incoherent run down, trying my best.

Function GetSerialNumber(Optional FilePath As String, Optional FileName As String) As String

  Dim DefaultName As String
  Dim DefaultPath As String
  Dim FSO As Object
  Dim SeqNum As Variant
  Dim TxtFile As Object
  Dim TargetCell As Object
  
    DefaultPath = "C:\Documents and Settings\bethan\My Documents\Scripting\Delivery Note Job"
    DefaultName = "Invoice"
    
      FilePath = IIf(FilePath = "", DefaultPath, FilePath)
      FilePath = IIf(Right(FilePath, 1) <> "\", FilePath & "\", FilePath)
    
      FileName = IIf(FileName = "", DefaultName, FileName)
      FileName = FilePath & IIf(InStr(1, FileName, ".") = 0, FileName & ".txt", FileName)
    
      Set FSO = CreateObject("Scripting.FileSystemObject")
    
       'Open the file for Reading and Create the file if it doesn't exists
        Set TxtFile = FSO.OpenTextFile(FileName, 1, True, 0)
      
       'Read the serial number
        If Not TxtFile.AtEndOfStream Then SeqNum = TxtFile.ReadLine
        TxtFile.Close
      
       'Update the serial number
        Set TxtFile = FSO.OpenTextFile(FileName, 2, False, 0)
        SeqNum = Format(IIf(SeqNum = "", "1", Val(SeqNum) + 1), "000000")
        TxtFile.WriteLine SeqNum
      
   TxtFile.Close
   GetSerialNumber = SeqNum
   
     
     With ThisWorkbook.Sheets
            With Cells(7, 11)
                If IsEmpty(.Value) Then
                    .Value = Date
                    .NumberFormat = "dd mmm yyyy"
                End If
                End With
 Visible = True
   Set FSO = Nothing
   Set TxtFile = Nothing
   End With
   End Function
   
Function MinusOne(Optional FilePath As String, Optional FileName As String) As String

  Dim DefaultName As String
  Dim DefaultPath As String
  Dim FSO As Object
  Dim SeqNum As Variant
  Dim TxtFile As Object
  Dim TargetCell As Object
  
    DefaultPath = "C:\Documents and Settings\bethan\My Documents\Scripting\Delivery Note Job"
    DefaultName = "Invoice"
    
      FilePath = IIf(FilePath = "", DefaultPath, FilePath)
      FilePath = IIf(Right(FilePath, 1) <> "\", FilePath & "\", FilePath)
    
      FileName = IIf(FileName = "", DefaultName, FileName)
      FileName = FilePath & IIf(InStr(1, FileName, ".") = 0, FileName & ".txt", FileName)
    
      Set FSO = CreateObject("Scripting.FileSystemObject")
    
       'Open the file for Reading and Create the file if it doesn't exists
        Set TxtFile = FSO.OpenTextFile(FileName, 1, True, 0)
      
       'Read the serial number
        If Not TxtFile.AtEndOfStream Then SeqNum = TxtFile.ReadLine
        TxtFile.Close
      
       'Update the serial number
        Set TxtFile = FSO.OpenTextFile(FileName, 2, False, 0)
        SeqNum = Format(IIf(SeqNum = "", "1", Val(SeqNum) - 1), "000000")
        TxtFile.WriteLine SeqNum
      
   TxtFile.Close
   GetSerialNumber = SeqNum
   
     
     With ThisWorkbook.Sheets
            With Cells(7, 11)
                If IsEmpty(.Value) Then
                    .Value = Date
                    .NumberFormat = "dd mmm yyyy"
                End If
                End With
                 
   Set FSO = Nothing
   Set TxtFile = Nothing
   End With
   End Function
   
  
Function CheckIFVoid(Optional FilePath As String, Optional FileName As String) As String

Dim MyFileCount As Integer
Dim DefaultPath As String
Dim TargetCell As Object
Dim Execute As Variant

   DefaultPath = "C:\Documents and Settings\bethan\My Documents\Scripting\Delivery Note Job"
    Set TargetCell = Sheets(1).Range("K5")
    
With Application.FileSearch
.NewSearch
.LookIn = DefaultPath
.FileName = TargetCell

 MyFileCount = 0
  If .Execute() > 0 Then
  Sheets(1).Range("K5") = MinusOne()
  End If
 If .Execute() = 0 Then
 MsgBox ("nothing found")
 End If
  End With
End Function

 Sub Auto_Open()
If Cells(5, 11).Value = "" Then
  Sheets(1).Range("K5") = GetSerialNumber()
  End If
If Cells(5, 11).Value >= 0 Then
Sheets(1).Range("K5") = CheckIFVoid
  
  End If
End Sub

Any help or guidance on this issue would be amazing!