Okay a friend gave me her laptop to see what's going wrong with it.
I found out that the hard drive is pretty much dead(0% smart health) reallocated sectors etc.
Laptop is a dell inspiron 15 which came with win8 preinstalled and she updated to windows 10.
Now, I've read here and there that windows 10 product key is tied to motherboard/cpu and its stored on uefi partition.
So the only thing i have to do is to install windows 10 on the new hard drive and it should be autoactivated?