I had same problem with new install of Win 7. After looking at fixes on sites I went to
the Device Manager and inspected the devices for power management tabs. My Logitech
USB keyboard had a power tab and I unticked the wake up box.( This prevents the start up from keyboard, it seems). The USB mouse also had a wake up box but I left it ticked so that the mouse could be used as the 'wake up' medium. So far sleep is working as planned. The BIOS, Network and many other bits can also cause 'sleeplessness' as well.