Hi,
The most likely culprit is some software you have installed under your user profile, most likely something that runs on its own. This could be antivirus software, system maintenance software or, if you are connected to an always on broadband connection, some software automatically look for updates. This type of activity could prevent the system from going to sleep and/or prevent the monitor from shutting shutting off as such things would be detected as the system being active instead of idle. It could be something as simple as a wireless mouse.
I would like to suggest that check the following:
1. If you are using Microsoft wireless mouse, please refer to the following KB and install the updates which mentioned in it:
The screen saver does not work when you use a Microsoft wireless pointing device on a Windows Vista-based computer
http://support.microsoft.com/kb/911895
2. Go to Control Panel\Hardware and Sound\Power Options\Edit Plan Settings, click Change Advanced Power Settings. Check Multimedia settings, change the option to "Allow computer to sleep". Please also adjust the other options as you need.
3. Disable all startup items and third party services when booting. This method will help us determine if this issue is caused by a loading program or service. Please perform the steps as the below KB:
How to troubleshoot a problem by performing a clean boot in Windows Vista
http://support.microsoft.com//kb/929135
If the problem still persists, please run the command: powercfg -a and paste the result back here.
Thanks.
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Sincerely,
Guozhen Wang
Microsoft Online Community Support