Sounds like you're dealing with a variety of issues.
First up, sleep can mean different things. I suggest you read up on the ACPI power states. Generally, what we think of sleep is the S3 state, where everything is turned off except for the RAM, where the current machine state is maintained. The system will appear to be totally powered off (except maybe for a blinking light on some systems), but if you remove the power source (uplug the computer and remove the battery if it's a laptop), it will lose its state and you'll have the restart the system from scratch.
The people in this thread are mostly discussing hibernation, which is the S4 state, in which the state is saved to disk instead of to RAM and the RAM can thus be powered off. Unlike S3, S4 can survive the complete loss of power, but in most cases, S4 looks a lot like S3, which is to say it looks like the system is off.
But sleep does not necessarily mean S3. Some systems, like an old server that I have, do not support S3 and instead use S1 or S2, where basically the CPU is paused, but the rest of the system does not necessarily stop. So fans and hard drives keep on spinning, power usage still remains pretty high (given how little modern CPUs consume when idle, S1 is not very different from just leaving the computer idle, and resuming from S1 is virtually instantaneous). So it sounds like you are either not entering sleep at all, or more likely, you're just entering S1 sleep. Windows does not control which sleep state you enter--that's controlled by the BIOS. Modern BIOSes default to S3 (aka STR) for sleep, but it is possible to pick S1 or S2 as the sleep mode. Make sure your sleep mode is configured for S3 in BIOS.
As for the system waking up, there are two likely sources. First, there are scheduled tasks that can wake the computer. For example, if you told Media Center to record a TV show at a certain time, Windows will wake the computer for that task. You can disable all forms of task-scheduled wakeups by editing your current power profile, going into the advanced settings, and turning off wake timers under the sleep section (by default, wake timers are disabled for laptops, and enabled for desktops). You could also revoke wakeup permissions on a per-task basis, but that requires tediously combing through the task scheduler.
The other likely culprit is Ethernet. Wake-on-LAN is supposed to wake the computer only upon the reception of a rather specific "magic packet", but many NICs today (I'm looking at you, Realtek) are configured by default to also wake on "pattern match", which appears to wake the computer if something on the network tries to contact it, which, IMO, is really stupid--if I want to wake up a computer on the network, I'd like to do it explicitly with a traditional WOL magic packet. So if there's an option for pattern match wakeup in your NIC's advanced settings, turn that off, and also be sure to check the box under power management to wake up only upon the reception of a magic packet (or disable wakeup entirely). Disabling pattern match wakeup does not adversely affect traditional magic packet Wake-On-Lan (I'm a very heavy user of WOL, and I have pattern match turned off on every system).