Scripting

first the disclaimer: I'm new to installshield, and vbscript, not new to scripting (go autoit!)...
I've got a feeling this is a real newb type question, and that I maybe should be posting in the Package Development forum (feel free to slap me if that's the case)

I've got a vbscript, that on it's own works, just copying a file before the install, I've pasted it into a custom action and it doesn't work. I get the 1720 error. when I look at the application eventlog, it tells me the problem's at line 20, that's where I'm instantiating the "scripting.filesystemobject" object. Can I not use the FSO from within a custom action?

the error from eventlog:
Product: Petty Cash.TEST -- Error 1720.There is a problem with this Windows Installer package. A script required for this install to complete could not be run. Contact your support personnel or package vendor. Custom action PSE_BACKUP_PETTYCASH script error -2146827788, Microsoft VBScript runtime error: Variable is undefined: 'WScript' Line 22, Column 1,

the script:
'==========================================================================
'
' VBScript Source File
'
' NAME: backup_petty_cash.vbs
'
' AUTHOR: Emmanuel Pleshe , Puget Sound Energy
' DATE : 4/5/2005
'
' COMMENT:
'
'==========================================================================
Option Explicit
Dim objFSO, sSource, sDest, iDateTime

iDateTime = (Hour(Now)) & ":" & (Minute(Now)) & " " & (Day(Now)) & "-" & (Month(Now)) & "-" & (Year(Now))
sSource = "C:\program files\SAP Tools\pettycash.xls"
sDest = "C:\program files\SAP Tools\PettyCash Backup " & iDateTime & ".xls"

'WScript.Echo sDest

Set objFSO = WScript.CreateObject("scripting.filesystemobject")
If (objFSO.FileExists(sSource)) Then
objFSO.CopyFile sSource, sDest, 1
End If



If the FSO just won't work, what's the best alternate route? Learn InstallScript?

Thank you in advanced for any help,

Emmanuel