If it is hardware-related problem, unplug/remove everything except mobo/cpu/ram/video and try to boot. Than plug devices one by one untill you find a problem.
In case this is software problem, i.e winxp is damaged to the point it cannot start in safe mode, than you need to:
1) In case you have precious data on boot partition, just put WINXP bootable cd, refuse to rapair existing xp install when prompted, create new partition (or use any existing one except for old winxp partition) and let winxp istall there
2) same as 1) only completely remove all partitions, or re-format existing winxp partition and put new winxp there.