In safe mode try
Repair XP by replacing corrupt files
in the run box, press enter and let system file checker find and replace missing / corrupt file in windows and explorer.This will take awhile to complete and you may be asked for the CD to be in the drive if windows is not preinstalled on the hard drive.
sfc /scannow problems
If that doesn't work then it may be a corrupt driver causing the problem.
Again in safe mode disable the drivers one t a time untill it will boot normally then update / reinstall the offending driver.