Hi,

Put the hard disk in the computer where it does not boot. then remove any other hard ware that it has in there e.g sound cards extra USB ports wireless cards, also unplug everything from the back and only leave the mouse, screen, keyboard and power.

See if it boots, if it does then put one thing back in the computer until you find the part that is not working,

You said it booted in to safemode, what was this about 30 seconds? when it is in safe mode go to device manager and update any drivers that need to be, the hardware with a "?" needs the drivers installed

please load xp again and press f 6 when you see it on the screen to load sata devices .
sata softwares and drivers are required in certain computers to load windows so while loading you will be asked to load those drivers through a floppydrive, so u must have
1)sata raid drivers (according to the hard ware installed in hp.)this piece of hard ware will not allow xp to boot if you dont give its drivers through a floppy drive(during installation and setup of windows.
so u must now have
1 floppy drive(usb)
2 a floppy disk with sata drivers
3)usb port to attach the usb floppy drive
after loading xp you will see xp running trouble free.
a easy solution is to load vists and the drivers are already there in vista cd.