XP will not boot
If your computer is an OEM unit from one of the large manufacturers, there should be an option for you to perform a rescue or restore of your system. the key press needed to gain access to the rescue/restore utility may be displayed when the OEM splash screen appears. If it doesn't, you'll need to go to the manufacturers website (or google) to find the correct key press needed.
IF you can gain access to the rescue/restore utility, select the restore option. If you had your OS set up to monitor and create restore points, all you have to do is select a resort point dated just before you started having bootup problems. Let the system complete the restore process, and your problem should be solved without losing any of your data. If that doesn't solve the problem, just perform the same restore process to another restore point that's even earlier than the restore date that was just performed.