How can I make WinXP give my Logoff script more time to complete?

My Logoff script times out too soon on certain slower machines, although it
runs fine on our faster machines--the script takes time because it involves a
scan of all files.

At my place of work, all users are running xp pro on their individual
machines, and they log onto a domain managed by a WinNT 4.0 server. Since
WinNT 4.0 does not support logoff scripting via it's group policy feature, I
put a copy of my program on each machine and configured it to run as a local
group policy via the gpedit.msc utility. The program ran, but it kept timing
out after the default 600 seconds or 10 minutes, so I changed the Maximum
Wait Time for Group Policy Scripts to 7200 and also enabled synchronous
running. This has not helped (although the program does run to completion on
the newer faster machines on which it needs less then 10 minutes to do its
job).

Any ideas on why it stills keeps timing out after 10 minutes?
So far I have configured the machines as follows using gpedit.msc:
User Configuration\Windows Settings\Scripts (Logon/Logoff) - ADDED MY logoff
script which does indeed start running at logoff
Computer Configuration\Administrative Templates\System\LogON - Run logON
scripts synchronously - ENABLED
User Configuration\Administrative Templates\System\LogON - Run logON scripts
synchronously - ENABLED
Computer Configuration\Administrative Templates\System\LogON - Maximum wait
time for Group Policy scripts = 7200

Could the problem be that that the timeout is specified for logON scripts
only? (yet there is no separate place to specify it for logOFF scripts). Or
could the problem have to do with the fact that all users are logging onto a
WinNT 4.0 domain? Is there some setting on the WinNT server side that can
override the timeout period I'm setting on the xp machines?

Perhaps WinXP Pro just does not allow logoff scripts to run more then 10
minutes. If so I'd like confirm this.

Thanks