This seems to do the trick. Let me know how it works and I'll post it to the UDF forum...

Code:

; ======================================================================
; 
; fnCompress - Uses WMI to compress files or folders
; 
; NAME: fnCompress()
; 
; AUTHOR: Christopher Shilt (christopher.shilt@relizon.com)
; DATE  : 5/9/2004
; 
; COMMENT: 
; 
; ========================================================================
Break On
 
fnCompress("D:\Target\tetrix") ?
;fnCompress("D:\Target\6to4svc.dll") ?
@ERROR " | " @SERROR ?
 
Function fnCompress($sTarget,Optional $sComputer)
    Dim $objWMI,$colTarget
    $sTarget = Join(Split($sTarget,"\"),"\\")
    If Not $sComputer $sComputer=@WKSTA EndIf
    $objWMI = GetObject("winmgmts:{impersonationLevel=impersonate}!\\"+$sComputer+"\root\cimv2")
    If @ERROR<0 Exit VAL("&"+Right(DecToHex(@ERROR),4)) EndIf
    If GetFileAttr($sTarget) & 16
      $colTarget = $objWMI.ExecQuery("Select * from CIM_Directory where name = '" + $sTarget + "'")
      $fnCompress = 3
      For Each $sTarget in $colTarget
         $fnCompress = $sTarget.Compress
      Next
    Else
      $colTarget = $objWMI.ExecQuery("Select * from CIM_Datafile where name = '" + $sTarget + "'")
      $fnCompress = 2
      For Each $sTarget in $colTarget
         $fnCompress = $sTarget.Compress
      Next
    EndIf
    Exit($fnCompress)
EndFunction