Preventing Standby

Tech-Archive recommends: Fix windows errors by optimizing your registry

Previously, my machine would go into standby at the preset time. But
now, sometimes it does not. The PC also does not turn off the screen
at the preset time.

I wonder what would prevent the machine from going to standby ? What
would reset the timer so that it does not kick off the standby mode.
Hard disk access ? Lan Activities ? Keyboard activities ? Mouse
activities ? CPU utilisation ?

Thanks for any advice !

.