I found this in a few different places around the 'net - hopefully it's helpful:
My Computer Won't Shut Down Itself After Installing XP
There are a number of users who have been complaining that their PC will no longer automatically power down/shut off without pressing the power off button on the computers unlike in Windows Me/95/2000. There could be a number of reasons for this, but the main one seems to be that ACPI is not enabled on the computer or in Windows XP:
- Click Start > Control Panel > Performance and Maintenance > Power Options tab.
- Then click APM > Enable Advanced Power Management Support.