See if this does it.
Code:
Option Explicit

Dim FSO, FLD, FIL, TS
Dim strFolder, strContent, strPath
Const ForReading = 1, ForWriting = 2, ForAppending = 8 

	'Change as needed
	strFolder = "C:\Documents and Settings\Mark\Desktop\VBS Sample"

	'Create the filesystem object
	Set FSO = CreateObject("Scripting.FileSystemObject")
	'Get a reference to the folder you want to search
	set FLD = FSO.GetFolder(strFolder)
	
	'loop through the folder and get the files
	For Each Fil In FLD.Files
		'Open the file to read
		Set TS = FSO.OpenTextFile(fil.Path, ForReading)
		'Read the contents into a variable
		strContent = TS.ReadAll
		'Close the file
		TS.Close
		
		'Replace the tabs
		strContent = Replace(strContent, vbTab, " ")
		
		'Open the file to overwrite the contents
		Set TS = FSO.OpenTextFile(fil.Path, ForWriting)
		'Write the contents back
		TS.Write strContent
		'Close the current file
		TS.Close
	Next
	
'Clean up
Set TS = Nothing
Set FLD = Nothing
Set FSO = Nothing