Tidied up the code a little.

This may not be the best solution as you didn't actually say what the script is being used for, but it is a literal solution for your outline requirement.
 Code:
Break ON

$=SetOption("Explicit","ON")
$=SetOption("WrapAtEOL","ON")
$=SetOption("ANSI","ON")

Dim $sDebug $sDebug=1			; Set to 1 to display debug messages, 0 to suppress
Dim $sFolderPath,$sInputPath,$sOutputPath, $sLastPath
Dim $iPick,$fso,$oInputFile,$oLastFile
Dim $asInputList, $sLastList, $iChose

$sFolderPath="D:\TEMP\"				; Folder containg data files
$sInputPath=$sFolderPath+"InputList.txt"	; File containing list to select from
$sLastPath=$sFolderPath+"InputList.old"		; Last week's file (copy of above
$sOutputPath=$sFolderPath+"OutputList.txt" 	; Selected items
$iPick=5					; Number of items to select

$=SRND(@MSECS)
$=RND(0)

; Copy last week's selections to backup file
Copy $sOutputPath $sLastPath

; Load up the original list
$fso = CreateObject("Scripting.FileSystemObject")
$oInputFile = $fso.OpenTextFile($sInputPath,Not 0)
$asInputList=Split($oInputFile.ReadAll,@CRLF)
$oInputFile.Close

; Load up last weeks selections
$oLastFile = $fso.OpenTextFile($sLastPath,Not 0)
$sLastList=@CRLF+$oLastFile.ReadAll+@CRLF
$oLastFile.Close

$=Open(1,$sOutputPath,5)					;Generating output file list

While $iPick
	$asInputList=Split(udfSqueeze(Join($asInputList,@CRLF),@CRLF),@CRLF)
	$iChose=IIf(UBound($asInputList)>0,RND(UBound($asInputList)),0)
	If $sDEBUG "Selection "+$iPick+": "+$asInputList[$iChose]+@CRLF EndIf	; *DEBUG*
	If $asInputList[$iChose]=""
		If $sDEBUG "   Input list exhausted - aborting."+@CRLF EndIf	; *DEBUG*
		Exit 259
	EndIf

	; If the item was previously selected then abandon it
	If InStr($sLastList,@CRLF+$asInputList[$iChose]+@CRLF)
		If $sDebug "  Selection dropped as used last week"+@CRLF EndIf	; *DEBUG*
	Else
		$=WriteLine(1,$asInputList[$iChose]+@CRLF)	
		$iPick=$iPick-1
	EndIf
	$asInputList[$iChose]=""
Loop
$=Close(1)

Function udfSqueeze($s,$q)
	$udfSqueeze=$q+$s+$q
	While InSTR($udfSqueeze,$q+$q)
		$udfSqueeze=Join(Split($udfSqueeze,$q+$q),$q)
	Loop
	$udfSqueeze=Split($q+$udfSqueeze+$q,$q+$q)[1]
EndFunction