Often times when I have Windows XP installations which will not boot one of the first things I will try is a check disk from the recovery console. This will often times fix Windows file problems which are preventing Windows from booting properly. In order to do this you will need a Windows XP installation disc.
Occasionally I get a PC in my shop which has a broken or non working cd drive. One of the mail tools I use is a disk called Hirens Boot CD. Yes I know its considered Warez however it is by far the best collection of tools for a PC tech around so I use it all the time. I will leave finding the actual disc up to you however once you have it you may want to create a USB drive with Hirens for situations when you dont have your cd or the cd option is unavailable.