said by koitsu:If the PC is asleep at that time, these tasks won't get run.
This is not necessarily true; scheduled tasks can wake up the machine. Mine wake up in the wee hours to do backups (via Home Server: task is scheduled on client) and to run Windows Update.
I took no particular action to make this happen: it's just normal. It seems like it would be pretty silly to write a scheduled task that was intended to run when no-one has been at the machine for a couple of hours, and then have it *not* do a wakeup. Self-defeating design.
I'm pretty sure that Security Essentials also wakes the machine: the PC *will* be asleep by the time MSE needs to run, and MSE *does* run, though I suppose there is a sliver of a chance that the PC just happens to be awake after doing something else.
EDITED: for every scheduled task, the 'conditions' tab in Task Scheduler has a checkbox for 'wake the computer to run this task'. I'm not at home right now so can't check on specific tasks.
I think that after being woken up, the normal rules for idling apply.