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It would help to know why you need the computer to shutdown?
Otherwise I'd recommend using an S3 (STR) suspend. Software can put the computer down in to an S3 and task scheduler can wake the computer from it. The computer uses marginally more power (1-5 Watts in my experience depending on the comp) than being in a Soft-off (S5) state.
The kicker is as long as your hardware supports S3, you don't need any extra software other than what XP comes with.