se o usuário for usuário convidado, você pode permitir um tempo específico para fazer login na conta, caso contrário, duas opções vêm à mente
1) defina um protetor de tela para um horário específico com uma data e hora específicas
else
2) configuração com algumas preferências para esquema de energia
De acordo com este site
The first is to configure a screen saver with the required timeout and
a password requirement. This can be done under User
Configuration\Policies\Administrative Templates\Control
Panel\Personalization. The required settings are: 'Enable screen
saver', 'Screen saver timeout', 'Force specific screen saver' (this is
important because if the system has no screensaver configured this
won't work) and finally 'Password protect the screensaver'. The
setting names and paths here are based on using the GP management
console on Windows 7 or Windows Server 2008/R2 since best practice is
to edit GPOs using the most current OS in your environment. If using
an older version of the console, the same settings are available in a
different location (I think its under the display control panel but I
don't have an XP or 2003 system to verify).
The second option is to use a group policy preferences power plan (or
Power Scheme and options if using Windows XP) as Alan indicated. The
plan should be configured to cause the computer to sleep after 15
minutes (just turning off the display wouldn't lock the computer) and
set to require a password upon waking. This option is a little more
involved since it not only configures the computer to enter sleep mode
(which can cause delays and problems when waking) but also support for
group policy preferences (available with Windows XP SP3, Vista SP1 and
Windows 7).
Espero que ajude