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.
If you don't see a 'Performance and Maintenance' option in Control Panel, you've obviously switched Control Panel to 'Classic View'. In the taskpane at left of the Window, select "Use Category View" and the Control panel Menu System will reappear, giving you the 'Performance and Maintenance' option.