Hi,
I had for the las 3 months a problem of overheating with my computer, on Win7 as on Ubuntu 14 (dualboot). The processors temperature were reaching 90°C.
I thought it could be a BIOS problem so I updated it to A10 (proposed on DELL website).
Not sure if it crashed during the update, or it restarted automatically (part of the update).
 
The point is that now, every time I start my computer, no matter what I do (booting on Windows, Linux, or just go through the options via F2, the computer shuts off after more or less 30 seconds. It doesn't even have to time to over heat.
 
Moreover, I'm not sure my fan is working anymore.
 
Does anyone have any idea  

o/
 
Looks like you got a problem with heatsink, like dry thermal grease or dead fan.
The shutdown after 30 sec is a thermal protection of the CPU when it's reaching too high temp, a software update will not fix the issue.  
You need to ensure that your fan is working first (by using any monitoring soft), if yes unmount your CPU heatsink and change the old grease !
And yes 30sec is enough to overheat if the CPU/heatsink contact is bad.

I strongly agree with Origon X as 30 secs are enough to overheat the CPU. No CPU fan = short term CPU death
 
BUT : the PSU may be another cause of an emergency shutdown as it occurred to me past year. The PSU voltage wasn't regular and my power supply stage (motherboard) cracked up. Then my rig was shutting down every boot even before accessing the bios.
 
Have you some equal components to cross test them? (only if the thermal grease / fan is fixed and boot problem persist)

Thank you for your answers! I'll try to check this out ASAP.
 
Could you advise me any monitoring software to check the fan activity? Since I only have 30 sec to install it, and about the same to monitore it, I have to go straight to the right one :-) (Either for Ubuntu or Windows)
 
I'm not sure I get what you mean thepalakat about the PSU. Neither IT nor English is my mother tongue. But, for about 3 months too, I have a message when I start my computer like "It's not the appropriate adaptor...blablabla...". But it actually is!
 
I will let you know, thanks again.
 
Léo

HWInfo64 for example under Win, I don't know for Linux
BTW it's French here not Italian

Wait a minute, is that a laptop?    If it is, you may have encountered the infamous "AC power adapter is incorrect" issue.  
 
Some people had frequent warnings about inappropriate adapter or incorrect AC power adapter, those were caused by a dysfonction from AC/DC adapter (the plugpack)  
 
 
@Origon X : il voulait peut-etre dire "Information technology" n'est pas sa langue maternelle  

It looks like it is a power plug or a temperature issue. Try to put your computer in "Power saver" mode (in Windows) to force the CPU to run at low speed.

 
Oops