OK....
Re the old hard drive, theres not much you can do about that. At least some of the data has been salvaged.
The Windows XP serial should be on a sticker on the side of the case. Use your own XP CD with that code. You may have to ring Microsoft in the activation process but just tell them that you are using the CD that was supplied with the PC. Some quick thinking may be required... (been there done that)
Depending on what the spec of the PC was, you may struggle to find the required components due to age. When I deal with an old PC that requires replacement parts, I always tell the owner that I don't think it's wise to spend money on old technology and I price up an upgrade. In some instances the price difference between old and new isn't much.
That PC looks like m/atx to me. The height isn't much going by the size of the PSU and the 80mm fan duct.
Shop built PC's don't usually come with full size ATX boards anyway.
Hope this helps.