Overheating problems with Alienware M15x

should be given but do you clean the dust out of it? the cooling system is basically at its bare minimum so any restriction in airflow causes issues and even then , ambient temp has another big issue,  if cleaned out regularly .. try elevating the back of it or adding a cooling pad ..

Thats a good start, but it could be a multitute of different things, try taking of the back cover and cleaning the intake grill's and fans, make sure to disconnect the power supply and battery and hold down the power button for 10-15 seconds, then ground yourself by either a earth strap or touching a metal water tap, to clean easily and safely go down to the local computer shop and buy a can of compressed air the techs use to cleanout machine's, if the cleaning does'nt work, it may be the thermal paste is breaking down inside the videocard, it could also be conflicting software, outdated drivers or even faulty components, try downloading MSI afterburn and monitor what temp the CPU and GPU are reaching when system shuts off, that may give you an indication of weather you need component repairs or software replacement

well cpu  80c isnt abnormal its thermal throttle isnt till around 100C  laptop cpu is designed to handle more temp then a desktop

I have cleaned the grills and fans with compressed air, but doesn't seem to help much. I have used HWMonitor to measure the temperatures, and the problem is the CPU, the graphic card stays more or less between 45ºC and 60ºC, but the CPU starts at 55ºC and easily goes up to 75-80ºC as soon as I start doing something half serious.

I will try to get a cooling pad on monday and see how much that helps.

Check your power supply is'nt faulty, has this problem started since you installed any programs recently, if so, uninstall them, do you have decent anti virus protection, not a free one like AVG something that actually works, remember you get what you pay for, if you pay nothing for a anti virus program, you'll get nothing in return,lolhaha, what about update all your drivers, and not all from the dell website as sometimes their a bit old, especially for obsolete models, try downloading some updated drivers straight from the manufacturer, like nvidia, amd, intel.............................

If that does'nt work then a complete fresh install may be in order, going so far as to wipe the drive clean first, at least it will eliminate a bunch of other potential problems, if after all this your stil having trouble, i think it may be time for a tech to look at it

No virus here (checked with Nod32) and I haven't installed anything recently that makes me suspect of this behavior. I have also checked drivers and I'm up to date with everything as far as I know.

I suppose I'll start doing a backup to try the fresh install before going to Dell Tech Service :S

Thanks for the help!