Laptop turns itself off???? Advent 7035?

Have an advent 7035 laptop and the other day it kept turning itself off and then got the black screen so chose to do last start up that worked but when it went to load it just switched off. So decided to reformat the hard drive but when it gets about half way through that it switches off so havent been able to do that. Have dismantled it and cleaned any dust out of the fans, have also tried a different hard drive in the laptop and the same thing happens it loads up but then switches off. Am at a loss at what else to try any help please???