Sudden auto shutdown when doing the works

Hello,
i was running my laptop with XP, i changed into vista ultimate o/s. But it is not working good and in between it is shutting down when am doing any work. Now i wnat to go back for XP o/s to my laptop, but when my xp o/s is booting it is agian shutting down sudenly. so please get the solution me to get XP install in my laptop without shutdown sudenly....