Shut Down an XP System That Won't Shut Down Itself

On some computers, pressing the power button on the tower will not shut down all the way. Rather, it displays the “It is now safe to turn off your computer” screen. Friendly Computers found a few bits of advice to turn your power button into a true power button. Read more below…

For starters, the next time you boot your system, hop into the BIOS settings and look for something called (or related to) Advanced Power Management. It needs to be enabled.

That step alone may solve your problem. If not, try this:

1. Click Start, Control Panel, Power Options. (If you don't see Power Options, click Switch to Classic View.)

2. Click the APM tab. (Don't have one? See below.)

3. Select Enable Advanced Power Management Support, then click OK.

Now try to turn off your PC. If that didn't do the trick, you need to verify that your PC is ACPI-compliant (meaning it supports Advanced Configuration and Power Interface standards), and/or look for wayward device drivers that might be messing with the shutdown sequence.

Source: http://www.pcworld.com/article/163437/shut_down_an_xp_system_that_wont_shut_down_itself.html