I have searched, but cannot find the answer to this exact question. I found lots of info on downgrading... but I don't think that is what I'm really looking for. I have a friend who purchased a new PC from Wal-Mart which is running Vista. His old PC runs XP Home, so he has an XP disc. It was specially built, and has a retail edition. He is wanting to completely wipe his hard drive of Vista, and install XP Home on that one too. Er, actually, he wants me to do it. So my question is will the product key for his Vista be considered valid by the XP installation? I'm not even sure if all this is even legal to do.... He wants everything on his computer to be legit. Any help and or advice here would be great. Thanks.