Recently One of my Vista machines started suffering from insomnia.
 
Every time Vista would go into sleep mode it will power down but will immediately wake up after a couple of seconds.
 
A little search in the windows system event log produced the following event: EventID 1, Wake Source: Device -USB Root Hub.
 
A little further search on Google landed me in a Microsoft KB that might help some one else but did not solve my problem:
 
A computer that is running Windows Vista appears to sleep and then immediately wake.
 
Although the above KB did not solve my problem, it came pretty close.
 
As it turns out some devices do not behave very well with power management turned on.
 
In my case it was a USB mouse.
 
To solve my problem I had to disable the mouse ability to wake up the computer from sleep mode.
 
In your case it might be another connected device.

If you want to disable a device's ability to wake your computer from sleep mode follow the steps below:
 
 1. Click Start, right-click Computer, and then click Manage.
 
 2. In the User Account Control dialog box, click Continue.
 
 3. Click Device Manager, expand the device family in question (in my case it was "Mice and Other Pointing Devices").
 
 4. Right-click the device, and then click Properties.
 
 4. Click the Power Management tab, uncheck the "Allow this device to wake the computer" check box, and then click OK. 
 
Hope this helps some one else...