I have a new Windows 7 PC that wouldn't sleep (not just automatically, but also when specifically told to). The screen goes black momentarily, but within two seconds the machine comes back as if nothing has happened.

I tried powercfg energy. This produces some errors quoted at the bottom of this post, plus some warnings about timer resolution. There are no USB devices connected other than wireless keyboard + mouse (Logitech MK250); I tried unplugging them to no effect. The motherboard is Asus P7P55D-E.

powercfg lastwake says "Wake History Count - 0", which I take to mean that it never actually went to sleep.

I dual boot into Ubuntu, and was having exactly the same problem on the Linux side. That turned out to do with USB 3.0, which I've now disabled in the BIOS. This has solved the problem on the Ubuntu side of things, but made no difference to Windows 7.

Any suggestions?

Suspend:USB Device not Entering Suspend
The USB device did not enter the Suspend state. Processor power management may be prevented if a USB device does not enter the Suspend state when not in use.
Device Name Generic USB Hub
Host Controller ID  PCI\VEN_8086&DEV_3B34
Host Controller Location    PCI bus 0, device 29, function 0
Device ID   USB\VID_8087&PID_0020
Port Path   1

USB Suspend:USB Device not Entering Suspend
The USB device did not enter the Suspend state. Processor power management may be prevented if a USB device does not enter the Suspend state when not in use.
Device Name USB Root Hub
Host Controller ID  PCI\VEN_8086&DEV_3B34
Host Controller Location    PCI bus 0, device 29, function 0
Device ID   USB\VID_8086&PID_3B34
Port Path   

USB Suspend:USB Device not Entering Suspend
The USB device did not enter the Suspend state. Processor power management may be prevented if a USB device does not enter the Suspend state when not in use.
Device Name USB Composite Device
Host Controller ID  PCI\VEN_8086&DEV_3B34
Host Controller Location    PCI bus 0, device 29, function 0
Device ID   USB\VID_046D&PID_C52E
Port Path   1,8