Installing speed fan is not likely to help you at this point.
I would figure out how to open up your case. Blow it all out. Check that all fans are working. (If possible). Then see if your computer will start. If not..........you're probably screwe'd. In otherwords, you need a new processor, motherboard etc. (Read..New computer)
Pay special attention to the processor. Don't remove anything other than dust. (Once into your case).
Your computer is designed to shut itself down when it overheats. If you are REALLY lucky....and haven't tried to re-start it too much once it was overheated...........cleaning it will fix your problem. This is assuming your fans are working.
Cleaning will also help ensure that it is not other things causing your problem. Blow compressed air on EVERYTHING. Get all the garbage out. Pay attention to the power supply as well. If it is plugged, it can dramatically reduce airflow.
Just do a good job. Then, do it again. If you're paranoid like me...................well you get the idea.
It doesn't do much good to call a job "done" if you are sitting there with half a can of air and "dust Bunnies" hanging out in, and around parts and vents. You may as well use it.
Don't touch anything internally.......and don't tilt the can. Use a anti-static wrist strap if you have one available. (Cheap, $5.00 US)
This is easier than it sounds. The hard part was asking for help. You're half way there!
If all goes well, speed fan is a good idea at this point.
P.S. I think Grim Reaper meant to type "MB" not "NB">meaning the mother board is overheating. I could be wrong though.