I don't know if XP will install over a Vista program. I'm not positive about the XP over Vista issue, but whenever I've tried to install an older O.S. than is currently installed on that drive, it won't install. I don't know why Microsoft does this but it will tell you "you are trying to install an older O.S. than your current O.S." You may need to put your Vista on another drive. Good Luck.