Update: A new fix for this issue has been discovered. Try removing third-party items from the system tray, then restarting. One user found that removing the OpenOffice 3.1 Quickstarter from the system tray resolved the issue.
Some users are unable to put their systems to sleep after updating to Windows Vista SP2, despite proper power management settings. There are a number of potential fixes for this issue.
Check media settings Sharing of certain files, specifically media files, can sometimes prevent proper sleep.
First, in Windows Media Player, check for options to share (in Tools > Options > Library). Eliminate these, then re-try sleep.
In the Network and Sharing Center, click ‘media sharing’ and turn off any options listed there.
Also, in power options, navigate to advanced settings, then check the settings in ‘multimedia settings.’
Update your drivers Check manufacturers’ websites for the latest driver editions for any third-party devices that are connected to your system. Many manufacturers have updated their drivers for basic compatibility with Vista SP2.
Graphics card drivers are notorious for causing sleep issues, so run Windows Update or check the manufacturer’s Web site for any of these updates.
Disconnect external (USB, etc.) devices Aside from your keyboard, mouse, monitor and network connectivity, try disconnecting any external USB, FireWire or other peripherals then re-trying sleep. You can reconnect the devices one by one and check for compatibility issues.