Notebook / laptop switches itself off. What to do?

My notebook switches off after almost 2 minutes. Before that, the fan works 100% after approx. 30 seconds. I wanted to reinstall Windows 10. Before that, the notebook has flawlessly according to the dealer. No matter what I do (go to the Bois, try to install Win, execute UltimatBootCD) it always goes out. I think that unfortunately there's a hardware defect. And yes the device was cold and the fan is clean. So I rule out overheating. What do you say?

I'm glad that it is a Pentium 1 notebook with few other components… Seriously, there's more than one device on the market. If we're to help you, a start with hardware information would be made.