Re: Is it safe for me to use Windows if someone else already used it
If it's an OEM Windows the license agreement states it should not be transferred from one PC to another. So in other words, if it came with the computer from a manufacturer, then you're not supposed to move it. Can you move it? Well, that's a different story.
Also pretty much any license agreement says you shouldn't have the same software installed on multiple PCs, so if it's not OEM then technically you should remove it from the other one before you put it on the new one. And you will need the activation code. You can get to it in your Control Panel\System window. Just a FYI.