Windows 7 can't sleep (It locks instead)
Solution 1:
Check your power settings by running through this list (especially make sure the hybrid sleep mode is disabled):
http://www.vistax64.com/tutorials/63567-power-options-sleep-mode-problems.html
It is for Vista, but it applies to Windows 7 (to a degree).
Solution 2:
I managed to get my sleep working with the free software Sleep.exe. Be sure to use it with the /f switch