If you don't have a backup of that data, don't use it in that laptop as a primary drive. Buy another drive, and use that ex-external drive as a backup drive for the laptop. Just get another external case for it. Because if you clik on the wrong thing, you will end up formatting the drive during setup or wiping the partition info.
One of the top things I stress to everyone is if you are that worried about your data, don't mess with it. Put the drive aside and use another one. Is saving $50 worth losing your files? If it is, go ahead and use the drive. If not, get a different drive or at least copy your data into anothe drive before you use this one to install Windows on. I have rebuilt my computers a dozen times, never lost any...