On the dell support forums it says your hard disk is failing.http://en.community.dell.com/support-forums/disk-drives/f/3534/p/18845373/18968359.aspx#18968359
also there could be dust build up on the cooling heatsinks that need to be cleaned with compressed air (dust build up around the vents and heat sinks is usually what causes shut downs).
Thanks for replying soon. I would check whether dust build up on the cooling heatsinks or not. how would i back up my data if hard disk fails, because it is turned off after boot up? any idea around this please