Recently what I thought was a motherboard failure prompted me to replace my motherboard. I went from an Asus P5N-T Deluxe to a P5N-D. That's a slight downgrade but it was a good deal on Newegg. Anyways, after installing the motherboard I noticed problems with the Windows 7 sleep mode. As soon as the computer went into sleep mode, it would immediately wake back up. It didn't do this with the previous board. Luckily, there is a built-in command that can tell you more about why Windows wakes up from a sleep. I found this post which helped out a lot.
To paraphrase, there is a command called powercfg that can tell you certain details about sleep. powercfg -lastwake will indicate what was the last device to wake your machine. Here is some sample output from that command:
Wake History Count - 1
Wake History 
Wake Source Count - 1
Wake Source 
Type: Fixed Feature
In this case, I used the power button to wake the machine. Use powercfg -devicequery wake_armed to get a list of all the devices that could wake your system. Combine that information with -lastwake and you know which device name to disable. Use powercfg -devicedisablewake <devicename> to disable a device. For some reason, all of my USB devices were causing the system to wake so I shut them off. I don't have too much problem with using the power button to wake the computer especially when the alternative is to have to shut it off entirely.
One thing I never found much information about is what the sleep states are. In the BIOS, there are two ACPI suspend types listed: S1 & S3. This is backed up by what powercfg -a reports:
The following sleep states are available on this system: Standby ( S1 S3 ) Hibernate Hybrid Sleep
The following sleep states are not available on this system:
The system firmware does not support this standby state.
Doing some fiddling with the BIOS settings, I noticed that S1 will let the computer sleep but the case fans are still running. My case fans are not connected to the motherboard. I still have to press the power button to wake. It's hard to tell what S3 does distinctly because as soon as it goes to S3 it wakes up again. I would doubt that my troubles are because the P5N-D does not support S2. Having had troubles with switching motherboards before, that seems a more likely explanation. The powercfg command provides a suitable workaround.
Glad that the article has shed some light for you :) It's a bit more frustrating when the command returns with a USB controller as there are many USB devices connected to the motherboard most of the time
Hi, I had similar problem, instant wake up after sleep, 'Wake History Count - 0', no devices with permissions to wake up, tried all power settings, but solution for my ASUS EeePC was disabling "Instant ON" - this is asus software. I simply installed it and disabled that feature and now my EeePC sleeps like it should.
There is the soft: support.asus.com/Download.aspx