Scheduled Task(s) Wont Run

I am creating simple on a schedule (one time only) tasks which run fine if the time trigger is reached whilst the computer is on and I'm logged in, but fail to run if the computer is shutdown or asleep when the trigger time is reached, even with "Run task as soon as possible after a scheduled start is missed" checked. I am aware that missed scheduled tasks aren't ran immediately on start-up (10 min delay?) but they never run. There are no events regarding the tasks in event log and the last run status is "Never" so they just don't seem to be picked up as missed tasks.

I have unchecked all the idle/power options so the task should run whatever. I have tried checking the "Run with highest privileges" box and checking "Run when user is logged on" or "Run whether user is logged on or not" The tasks will run manually fine if I select run from "actions". At the moment I'm just trying to get a one-time scheduled task to display a message so I'm not trying anything fancy. Setting the task to run at logon or startup works fine. The system time/date is set Ok. A Google search revealed one guy with a similar problem but no solution. This is Vista HP sp2 with all important updates applied - running on an acer aspire laptop. A friends Samsung with the same Vista setup also shows this behaviour.