Hey,
a few possible problems:
1. Bad power connector
Check the cables and your battery.
2. Virus
If you dont have antivirus I suggest you get
www.avast.com download the home version.
3. Overheating
Does the laptop get VERY hot fast? monitor the temperatures.
If you are unable to solve the problem, id recommend to take it to a professional, it would be easyer for you and me.
Id recommend turning off automatic restart on failure.
Heres how to turn that off:
when windows is starting before the logo comes up, press f8 untill a menu shows up and select disable automatic restart.
if you do that you might start seeing some blue screens when your computer shutdown instead of just shuting down, these screens can be used to solve the problem if you get such a screen, type up the all the codes. Example 0x00000f1 (0x000000...)