I guess you could have air vents cluttered with dust. If you do not want to open your machine, turn it off and put running vacuum cleaner hose on the air INTAKES. Try to slide it around intake holes, one at a time, making "o" patern. If some dust "strings" will start coming out of those holes, continue doing that until it's gone. You can help yourself with tweezers to pull them out. If there's no dust, you may try clean installation of Windows. If that won't help, you should repaste your laptop. Eventually call Dell Customer Service and tell them that your laptop heats so much that it shuts down itself. They should send technician to do the job for you.