The following are things to try if you are having problems with your Vista/Windows 7 MCE sleep settings.
1. Goto Advanced Power Config and changed setting for sharing media to Allow computer to sleep
2. Make sure hard disk powers down before my machine goes to sleep i.e. set sleep to 10 minutes, then I see hard disk power down to 5 minutes
3. Check power settings on NICs especially nForce Network Controllers. Ensure the NIC can be switched off to save power and not wake the machine
4. The Windows 7 Media Center slide show screensaver can prevent sleeping. Disabled it in Media Center, it’s under the picture settings as it’s a photo slideshow screensaver)
5. There used to be a problem with homegroups in Windows 7. Being a member of a homegroup prevented going to sleep. Try removing it. (Control PanelAll Control Panel ItemsNetwork and Sharing CenterAdvanced sharing settings)