Option Explicit
'' необходима ссылка на библиотеку:
'' Microsoft Scripting Runtime
Sub TestForComma()

Dim a, b, s, fa, fb, fs, ForReading, fname
ForReading =  1 
fname = "data.txt" '' короткое имя!!
Set fs = CreateObject("Scripting.FileSystemObject")
Set b = fs.OpenTextFile("C:\data.txt", ForReading, False) ''исходный файл
Set a = fs.CreateTextFile("C:\data2.txt", True) ''файл временной копии

Do While Not b.AtEndOfStream
s = b.Read( 0 )
s = b.ReadLine
a.WriteLine (Replace(s, ",", ".",  1 , - 1 ))
Loop

a.Close
b.Close

Set fb = fs.GetFile("C:\data.txt")
fb.Delete
Set fb = Nothing

Set fa = fs.GetFile("C:\data2.txt")
fa.Name = fname

Set a = Nothing
Set b = Nothing
Set fa = Nothing
Set fs = Nothing

End Sub