Be sure all the heatsinks are where they belong, there isn't a lot of dust in there and the fans work like new.
If that's OK, then HDD failures are common. You can test the machine without the HDD.
My computer suddenly came up with a blue screen and when i tried to start it again the resolution was fixed at 800x600. And there were a lot of green dots on the screen. Naturally i assumed it was the graphics card, it even had a yellow exclamation mark in device manager. So i tried to reinstall graphics drivers, and when i tried to restart it was stuck in a boot loop of "Disk Read Error". I then opened the case to see if something was disconnected, and when i tried to turn it on again it said "Machine Check Error". I have also tried to clean the computer for dust, but it seems nothing is working. I know i have some defective hardware but i have no idea what it is. Because i tried taking out the GPU and turning on the pc and it still said "Machine Check Error". I'm starting to think it might be the CPU, Motherboard or the RAM. Right now it says Disk Read Error, but it is somewhat interchangeable. Any help is appreciated