A system file can be damaged in some way or another that makes it unable to start. I would suggest running sfc /scannow to let xp repair or replace the system files that may be damaged. If no joy you might consider doing a short hop restore. This means just go back in hours not days. Or do a last known good configuration.
If you have a Windows XP install disc do the following -
Insert Windows CD run sfc /scannow or do a repair install.