First off: Windows assigns drive letters arbitrarily.
A: will always be your floppy. C: will most always be your Windows Operating System.
D: most likely your DVD/CD.
If you have additional drives or partitions then they will be E: F: G: etc.
What type of XP CD are you using? Is this a retail version Microsoft CD? Sorry if I have asked before.
Also do you know the manufacturer of the 70 GB hard drive?
The manufacturer will have a diagnostic that you can run off of it.
The type of CD that you are using is the most important thing to clarify.