I have n older Dell pc and am having trouble with windows XP. I do not have the original windows xp disc. I have a dell windows xp disk from a friend and tried to reinstal windows but it say I cannot as it is an older system than is installed. I cannot boot from my cd as it does not show up on the bootable devices.
No guarantee that it will work, but the problem is that your OS has been upgraded since the installation and you cannot install an older version.
So, what I would do is clean the disk. Download and run DBAN from HERE, burn to a CD or DVD and boot from it and nuke the drive (everything gets permanently erased). Once that is done, you should be able to install using your XP key.