Hi, My laptop turn off automatically during on first boot up. Typically happens every long period of not using it. Means every time I come back to my office. The brand of my laptop is Toshiba Portege M900. Please help. Thanks, Anonymous

Try cleaning the Air Intake Duct first.
The laptop is shutting off because it's dirty inside.

A Processor has a Thermal Limit. This is the maximum operating temperature it will operate at.
If the Thermal Limit is exceeded, the Processor turns off.
(BIOS turns it off)

This is a built-in fail safe system so that the Processor will not burn up.

The Air Intake Duct is on the bottom of the laptop, to the left of the,.......better still, let me show you,

http://howtosolvethat.blogspot.com/2010/07/how-to-disassembly-toshiba-portege-m900.html

(Click on the photo to enlarge)

Where it states - Slide the latch -, go to the label above it, the label above that, then to the vent grille.
Air Intake Duct.

ALL power removed from the laptop.
Remove the AC adapter (Charger) and Battery.

Attach the plastic straw provided to the nozzle of the can of air. Break the plastic lock tab off of the top of the nozzle.
You may wish to place a soft cloth, such as a towel on a table, then stand the laptop up on one side on the towel. (Towel, or soft cloth to prevent scratching the laptop, and table)

Hold the laptop with one hand, the can of air with the other. Apply jets of air to the Air Intake Duct grille. Slowly go back, and forth across the top of the grille opening, then drop down to the next opening, (Slot), and repeat.

Squeeze the trigger in all the way as you start, let go when you reach the other side. Apply air as a painter would apply paint with a paint gun.

Refrain from tilting the can of air as much as possible. Tilting the can makes more propellant than air come out. This results in the can of air's nozzle freezing up. The nozzle will eventually freeze up. The velocity of the air coming out freezes the moisture in the air.
When this happens sit the can of air down for a while. Let it warm up.

As stated due to the velocity of the air there will be moisture 'produced'. After you are satisfied that the Air Intake Duct is clean, let the laptop sit.
The moisture will dissipate. (10, 20 minutes)

Reinstall the battery, and AC adapter.
WAIT 1 minute, turn the laptop on.

Still no?
Looking at Page 8 of the above link, click on the photo at the bottom of the page.
(Page numbers are below the last photo on every page. Click on Page 8))

At the top right of the photo is the Fan Assembly. It's the silvery looking round object with three curved openings in it. (Some of the fan's blades can be seen through the opening)

Observe the flat copper tubing, and how it leads up to the Fan Assembly.

(Copper tubes are Cooling Tubes. They connect to a Heatsink sitting on top of the Processor,
and further to the left, to the graphics chipset. Part of the Processor's socket can be seen. The purple rectangle is one outside edge. The graphics chip is encircled in red)

Where the cooling tubes meet the Fan Assembly is a type of Filter. (Rectangular in shape)
This filter can become clogged, and using just a can of air from the outside of the laptop may not do the job.

The fan's blades, center hub, and surrounding cage, (Shroud) will also need to be cleaned.
The laptop would then need to be disassembled, and the clogging debris physically removed.
(Q-tips, small soft brush, and air)

Due to the high heat involved when the Processor is operating, (Graphics chipset too), the Thermal Paste may have dried up.

To regress;

Heatsink:
Typical construction of a Heatsink is a flat plate of metal with tall, thin fins protruding from it. The plate of metal absorbs heat from whatever object it is placed against, and the tall, thin fins absorb heat from the plate of metal.

The tall, thin fins then radiate the heat away. The fan pushes air over, and through the fins of the Heatsink, and the air helps carry heat away.

The bottom of a Heatsink, and the top of the Processor's case are not perfectly smooth. A magnified view would reveal 'Pits, Hills, and Valleys'.

These imperfections create air pockets in-between the surface of the Heatsink, and Processor.
Air is an Insulator. Not a Conductor.

Thermal Paste, (Also known as Thermal Grease, or Thermal Compound), fills these voids, and is an excellent conductor of heat.

Should you decide to disassemble your laptop please post in a Comment.
We need to discuss Anti-Static Precautions, providing labeled compartments for the individual screws, and proper disassembly techniques, for a start.

Reply in a Comment.
Regards,
joecoolvette



[ More disassembly information,

http://www.tim.id.au/blog/tims-laptop-service-manuals/#toc-toshiba

Go to the second Portege heading down.
Click on M900D
(This is a PDF file. Allow up to 30 seconds to download. Additional time may be required once you see the first page. Watch the download progress bar. There is no 'junk' nor malware attached]