a.Skip f.Size -  3 

Dim fso, a As Scripting.TextStream, f As Scripting.File, retstring, retstream
Const ForReading =  1 , ForWriting =  2 
Set fso = CreateObject("Scripting.FileSystemObject")
Set f = fso.GetFile("d:\config.sys")

Set a = f.OpenAsTextStream(ForReading, False)

'перейти на предпоследнюю строку
If f.Size >  0  Then a.Skip f.Size -  3 

Dim kluch As Boolean

Do While a.AtEndOfStream <> True
    retstring = a.ReadLine
    If Len(retstring) =  1  Then
        If Asc(retstring) <  32  Then
            kluch = True
        End If
    End If
Loop
a.Close

If kluch = True Then
    kluch = False
    Set a = f.OpenAsTextStream(ForReading, False)
    Do While a.AtEndOfStream <> True
        retstring = a.ReadLine
        If retstring Like "files*" Then
           retstring = "fileshigh=200"
        End If
        If Len(retstring) =  1  Then
            If Asc(retstring) <  32  Then
                kluch = True
            End If
        End If
        If kluch = False Then
            retstream = retstream & retstring & vbCrLf
        End If
    Loop
    a.Close
    
    Set a = f.OpenAsTextStream(ForWriting, False)
    a.Write retstream
    a.Close
End If

    Do While a.AtEndOfStream <> True
        retstring = a.ReadLine
        If retstring Like "files*" Then
           retstring = "fileshigh=200"
        End If
        If Len(retstring) =  1  And Len(retstream) >= (f.Size -  3 ) Then
            If Asc(retstring) <  32  Then
                kluch = True
            End If
        End If
        If kluch = False Then
            retstream = retstream & retstring & vbCrLf
        End If
    Loop