Hi James,
If you are having difficulty getting your XP disc to start there is a way you can install the OS on another machine and prepare it to go back into yours. First, connect your HD in the other machine then remove the power supplies to the existing hard drives in the other machine. This will ensure that your drive is assigned C:\. Install your OS then follow the instructions in
this article to run sysprep. Put the hard drive back in your machine and reboot. let us know how you get on.