If you know when it will be finished by you can try this...
Start > RUN > Type:
Shutdown -s -t 100
XP will shut down in 100 Seconds...Just alter the figure in seconds to what you want ...
2 and a half hours would be 9000 - It will display as hh:mm in the box that appears but you have to enter it in seconds.
Or click here and downlaodTweak XP this has a countdown, plus lots of more "things".