hmm.. it depends on the OS too...
under vista or XP make sure u use an UPS which comes with support for vista or XP for example APC 500 VA or 650 VA... in that case from power settings make sure system hiberbernates well before u run out of bettery... I would say hibernate at when 40% bettery is left

also make sure the Torrent client in at the start up object...
Now if the system cant hibernate autometically, then under ur OS you have to remove the user account password and enable the auto login !!! and have to make sure the Torrent client is at start up object...
Next comes the BIOS part... Go to the Power TAB of the BIOS, and in the ARM configuration,
Enable the Restore on the AC Power Loss...