I'm just pretty confused, trying to re-install Windows is harder than it should be.

[APotatoCalledJoe]

So, my HDD died on my personal PC a while ago, finally got a new SSD to replace it. I'm trying to re-install Windows using an 8.1 installation flash-drive my friend lent me, however I can't find any way to buy an 8.1 key. Also, I'd just get Windows 10 myself, however, I can't get the tool due to the home computer, the one I'm using now, being bought pre-used, therefore not giving me administrator access. Any help is appreciated, I've been struggling with this for an hour.

 

[SuperSoph_WD]

Hey there, @APotatoCalledJoe!

Unfortunately, the issue is definitely coming from the fact that you are trying to use a pre-used Windows installation. I'd strongly recommend you contact MS Support for assistance regarding getting a new Windows activation. What was the previously used OS on your computer?
If you haven't changed the PC (Motherboard), you shouldn't face issues re-installing the same genuine Windows OS that you had previously there with the old HDD. Here are some other useful MS articles you need to check out.

Best of luck! Hope this helps you!
SuperSoph_WD