How can I use %SystemRoot% in a script?

Uncle!  I'm not a programmer and someone else wrote this originally.  It's a script that runs weekly to do clean-up on our computers (this is just one section) Most of the machines are "clean" installs of WinXP... but... I have a few that are upgrades.  Therefore, the upgrade machines have "WINNT" as the SystemRoot name.  The script will, obviously, not run on these machines since the code references the path "C:\Windows....".  Is there a way to return the %SystemRoot% name and use it in the path shown in the code below.  Finding a way would add a few years to my lifetime as I have been searching for weeks.

Sub sbDefrag()
	set WshShell = CreateObject("WScript.Shell")
	Dim WshShell, Result, objFSO, objNetwork

	Result = WshShell.Popup("Weekly Computer Maintenance Is Being Preformed Please Do Not Log onto this Machine!  The I.T. Department.", 10, "Title", 65)

If result = 1 or Result = -1 THEN	
	set WshShell = CreateObject("WScript.Shell")

	'Runs a clean up on the C: Drive	
	WshShell.Run "cleanmgr /sagerun:100"
	WScript.Sleep 5000

	'Cleans out the WIndows XP Prefetch directory
	Const DeleteReadOnly = True
	Set objFSO = CreateObject("Scripting.FileSystemObject")
	objFSO.DeleteFile("C:\WINDOWS\Prefetch\*.*"), DeleteReadOnly
	objFSO.DeleteFile("C:\WINDOWS\Prefetch\*"), DeleteReadOnly

Select allOpen in new window