After a re-installation of Windows 7 Professional; a HP Compaq dc7900 Small Form Factor PC would not stay ‘asleep’. The Power settings were enabled to make the machine sleep but it would close down and then almost immediately wake-up again.
The solution was in the Network Adapter’s Advanced Property settings: Disable ‘Wake on pattern match’.
The Windows setting can be changed by following: Device Manager -> Network Adapters -> Properties -> Advanced
NOT the Power Management – Wake on LAN (WOL) / Magic Packet settings or the power-on settings in the BIOS which I tried first.