Glenn,

SP6a did not install or update WMI, if you have SMS it would have put at least an older version of WMI on the system. In order to use WMI on the NT 4 systems I'd recommend checking or updating WMI on those systems.

Windows Management Instumentation (WMI) CORE 1.5 (Windows NT 4.0)

Windows Management Instrumentation Downloads

Windows Management Instrumentation (WMI) SDK 1.5 (Windows NT 4.0 and 2000)

WMI Administrative Tools

Management (WMI) Extensions for Visual Studio .NET RTM Server Explorer

WMI - Windows Management Instrumentation - White Papers, etc...

Frequently Asked Questions - Windows Management Instrumentation

Microsoft Management Technologies Community Center

As far as the ShutDown process from KiXtart I have not used it enough to give any insight to the issues you may be having.

Don't forget we do have an FAQ here about rebooting. Original I think is here
Rob van der Woude's Shutdown and Reboot page

Jorgen Bosman writes a pretty nice utility as well for rebooting systems
Jorgen Bosman homepage

Poweroff is a small program for Win95/98/NT/W2K/XP to schedule a shutdown/reboot/logoff/poweroff/Standby/Hibernate/Lock/Wake-On-LAN at a certain time. I also support command line options that allows poweroff to be used in batch scripts. It has a built-in scheduler, it allows you to run a program before doing the action, you can give a warning message to the user, wait for a process to finish before doing the action and full remote control is supported.
PowerOff v3.0.0.16