Easy way: Schedule to shutdown your XP machine.

Yesterday night I have started to download iPod software and its estimated to complete in 30 min. It was late night and I didn't want to wait just for download to complete. I therefore searched for way where I can shutdown y machine automatically after 1 hour. I learnt that the Scheduler wizard cant handle auto shutdown so searched for other alternatives and found this one which works like a charm for me on Windows XP SP2 machine with administrator privileges:

1. open command prompt and type tsshutdn <> /powerdown. For Example, tsshutdn 1800 /powerdown will shutdown the machine after 30 min.

For more information about tsshutdn and its other options, refer to MSDN documentation.