IIt troubled be a lot, my windows XP is not at all rebooting/restarting . For a solution i googled for about 1 week and finally get in to a conclution tha it might be a virus.
It was a right guess and i replaced my existing AVASt with AVG free edition. it took hardly 1 hour to finish install and i gave a full scan to all m HDD drives
Finally about 50 infections removed and now my computer shutdown and restarts normally, thanks to AVG for such a brilliant solution that too for free!!!
download link : free.avg.com