Access the Device Manager (On this Win7 machine, it's Control Panel-Device Manager), then double click on the "Network Adapters" listing to show the specific network devices on the machine. RIGHT click on the adapter listed there, choose "Properties", then click on the "Power Management" tab. (if you also have a wireless adapter on the desktop, you'll need to check the wireless adapter settings also). Look through the list of options listed and UNCHECK any of the "Wake on LAN" items.
Hope this helps.
Grif
My HP Desktop won't stay in sleep mode. It gets what appears to be a ping from the network or router that wakes it back up. This is consistently happening from 15 secs to 40 secs after going to sleep. I see the port light on the router flash and the computer starts.
Likewise my Laptop with Vista stays in sleep mode when direct connected to another port on the same router.
Where in Vista should I look to address this???

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic