I know this is probably a silly question, but I rarely use Steam so I need some help. I was looking in a thrift store today and found a copy of Portal. Inside the case, there was a product code or something inside. I put the disc into my laptop when I got home, put in the code for it to install, but I got the message that said that someone else had already used the code on a different steam account. Is there any other way I can get the game to install?