Low power mode not working

I have Vista home premium and am having a

problem using the low power state. I go to the start menu and click on the

round icon to put the machine into the low power state. My computer will go

into low power mode but fires back up again almost immediately.

Microsoft Windows [Version 6.0.6001]

Copyright © 2006 Microsoft Corporation. All rights reserved.

C:\Users\Graham>powercfg -lastwake

Wake History Count - 1

Wake History [0]

Wake Source Count - 1

Wake Source [0]

Type: Device

Instance Path: ACPI\PNP0C0C\aa

Friendly Name:

Description: ACPI Power Button

Manufacturer: (Standard system devices