To make a long story short, my friend gave me his old gaming computer and told me that it needed a hard drive so my friend and I moved over the one from the PC I was using which was running on Windows 10. When I got my PC, it was running on 8.1, it was activated and I was able to upgrade with no problems. However, when I moved my hard drive over to this PC I began to see the little "Activate Windows" watermark in the bottom right corner of my screen so I gave my friend a poke and asked about it. "Windows is installed on your hard drive yes, but your serial key is on your motherboard is my theory is correct. So basically just re-enter the serial key you got with your other PC and all should be fine." So, I did exactly that. It worked for a couple days, but now I have the watermark again and I don't know why? It's also not taking that serial key anymore? If anyone could help me out with this, I would greatly appreciate it! tl;dr: Friend gave me PC. Moved hard drive over from my old PC. Re-entered my serial key to remove the "activate windows" watermark in the corner. It worked for a couple days but now it's not taking that serial key plus the watermark is back and I don't know why.