MGGP said:
Start, Run type SHUTDOWN -A and hit OK to stop Shutdown.
That is Abort a shutdown (that has been initiated).
In XP Pro there is a control over who can do it in a run of gpedit.msc,
on the Computer Configuration - Windows Settings - Security Settings -
Local Policies - User rights assignment
'Shut Down the System' is usually permitted to Administrators only, but
you could adjust this to something more specific.
If doing so I would also make sure that in Control Panel - Power options
the Advanced page 'When I push the Power button' is set to 'Do Nothing'