How to enable Sleep mode on Win 7

It can also be your display drivers. Open an admin mode cmd and do:

powercfg -a

If it complains about VGAPNP.SYS, you have your prime suspect right there.

It could be an issue with the Multimedia settings. Go to the "Power Options" control panel and underneath the active power plan, select "Change Plan Settings". From there, click "Change advanced power settings". From there, find the node "Multimedia Settings" >> "When sharing media" and from there select "Allow the computer to sleep" for the "On battery" and/or "Plugged in" states.

Media center apparently will disable sleep when it thinks that media is being shared.