The naming of my var $strFile in my code is wrong, because it is about an object, not about a string.
Maybe you better change it to $objFile
 Code:
;*************************************************************************
; Script Name: Find-_AC-Files
; Author: green78
; Date: 31/07/2007
; Description: Find and rename files with _AC in directory
;*************************************************************************

;Script Options
If Not @LOGONMODE
	Break On
Else
	Break Off
EndIf
Dim $RC
$RC = SetOption("Explicit", "On")
$RC = SetOption("NoMacrosInStrings", "On")
$RC = SetOption("NoVarsInStrings", "On")
If @SCRIPTEXE = "KIX32.EXE"
	$RC = SetOption("WrapAtEOL", "On")
EndIf

;Declare variables
Dim $arrFiles, $objFile
Dim $strPath, $strPrefix

;Initialize variables
$strPath = "C:\Test\Test\"
$strPrefix = "BG_STAT_"

;Code
;Only list hits that have _AC in their name (/M _AC)
;recurs subdirectories (/s)
;list only files (/A-D)
$arrFiles = DirPlus($strPath,"/S /A-D /M _AC")
For Each $objFile in $arrFiles
	;$objFile.Name ?
	;$objFile.Path ?
	;$objFile.ParentFolder ?
	;Only rename files that have not been renamed yet
	If Not Left($objFile.Name,Len($strPrefix)) = $strPrefix
		$objFile.Move($objFile.ParentFolder + "\" + $strPrefix + $objFile.Name)
	EndIf
Next

;UDF Section
;Copy the DirPlus UDF here