Computer clutter

Alright, so I got my computer back, but does it work? They replaced the damaged harddisk but the computer wouldn't start when I got it back. Finally after a couple of days with patient work the computer did start and even worked for a couple of days, before it was game over. Suddenly, and without no reason it shut down. When I try to restart the damn thing it wont. All I get is the message "startup repair tool recommended", when I accept it shut down again. No matter how many times I try it just wont work. The next day it might start working again, but shut down after short time! No way it will start again! I am so frustrated!!! My patience is gone, actually I have no patience when it comes to computers! My blood pressure is extremly high. I am sure that grey hairs are popping out of my head! All because of this damn computer!!!
It seems like they just replaced the harddisk and didn't bother to check if the computer worked before they returned it. I am beyond pissed!!! I am about to throw the damn machine out the window!!!