Windows XP: How to Change Power Options When No User is Logged on
HKEY_USERS\.Default stores the profile used when no users are logged on to the computer (such as when the CTRL+ALT+DELETE logon prompt is displayed).
HKEY_USERS\.DEFAULT\Control Panel\PowerCfg
Value Name: CurrentPowerPolicy
Data Type: REG_SZ
Value Data: 0 thru 5
0 - Home/Office desktop
This scheme is suited to most home or desktop computers that are left plugged in all the time.
1 - Portable/Laptop computer
This scheme is designed for extended battery life for portable computers on the road.
2 - Presentation
This scheme keeps the monitor on for doing presentations.
3 - Always On
This scheme keeps the computer running so that it can be accessed from the network. Use this scheme if you do not have network wakeup hardware.
4 - Minimal Power Management
This scheme keeps the computer on and optimizes it for high performance.
5 - Max Battery
This scheme is extremely aggressive for saving power.
Also:
HKEY_USERS\.DEFAULT\Control Panel\Desktop
Value Name: PowerOffActive
Data Type: REG_SZ
Value Data: 0 or 1
0 - Don't turn off monitor
1 - Turn off monitor