System Hibertante and Stand By upon a specified time

altogetherHello!

I own a laptop running Windows XP Professional. In the past, the Power
Schemes located in the Power Options Properties, which enable to configure
when the laptop goes into "hibernation" or "stand by" upon a certain time,
used to work fine. However, a few months ago they stopped working altogether.
When the specified time is reached, the computer simply turns off the monitor
for either configuration ("System standby After..." and "System hibernates
After...").

The other Power Schemes configurations are working pretty well ("Turn off
monitor" and "Turn off hard disks").

Such situation happpens either when the laptop is plugged in or running on
batteries. Is there any way to restore the behaviour of the options that
enable the computer to Hibertante or Stand By upon a specified time, so that
they can work properly again?

I am thankful for any help.

Kind regards,
Rui Nunes