Check if your pc is setup to work with ACPI

First, you may want to check out ACPI. Otherwise, follow the steps below.

On a Windows PC, Click on Start, then Open Control Panel. Open System.
Click the Hardware tab, then click Device Manager.
Click the Plus next to Computer. Here it should say ACPI Uniprocessor PC. If it doesn't, the only way to fix it is to reinstall Windows XP. You don't have to reformat the PC, just start to install Windows XP in the usual fashion, then when it prompts you, tell it to repair the windows installation. To make sure it installs the ACPI Uniprocessor PC properly this time, you will need to follow this guide.
If that is Ok, then Close Device Manager and System Properties. Goto the Control Panel as you did above, and this time Click on Display. Click on the Screen Saver Tab. Then Click on Power.
Now the Power Options Properties box appears. Click on Hibernate Tab. Make sure Enable Hibernation is checked.

Now click on the Advanced Tab. Click the down arrow indicated by the red arrow. Select either Shut down or Hibernate, depending on what you want the computer to do when the CarPC EZ or Pro Power Controllers tells the PC to shutdown.

Note: Hibernate usually works well on WinXP and is perferrable because it greatly decreases the boot-up time. I use Hibernate for my own CarPC, however, some usb devices have trouble resuming operation from a hibernate. Therefore, you may wish to choose Shut down.