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. Here is how to enable it: 
Click - Start - Control Panel - Performance and Maintenance - Power Options Tab 
Then click APM - Enable Advanced Power Management Support