since Windows 7 / Windows Server 2008 R2 this key does not exist by default.
Without configuring a default timeout via Group Policy, the system does not have a timeout and therefore does not start the screensaver.
There are 2 workarounds to solve this issue
1) Configure a Default ScreenSaver Timeout using Group Policy
User Configuration\Administrative Templates\Control Panel\Personalization\
Screen saver timeout
2) Use Group Policy Preferences to configure a new default value for
in a Group Policy applying to users.
User Configuration => Preferences => Windows Settings => Registry
New => Regsitry Item
Key Path: Control Panel\Desktop
Value Name: ScreenSaveTimeOut
Value Type: REG_SZ
Value Data: 600
Using Item Level Targeting (Common Tab of setting) the appliance of this setting can be restricted to systems runnign Windows 7 or WIndows Server 2008 R2