so to re-cap.
if you turn the computer on, nothing happens, you get a couple of beeps, and a blank cursor
if you start the computer with hirens boot disk in it then you can load the boot manager and tell it to boot from the hard drive, then windows loads fine.
so it does look like your computer just can't boot from the hard drive ordinarily.
e.g. you can boot from a CD, but can't boot from a hard drive.
I'd suggest starting the computer with the windows XP installation disk in it.
go into RECOVERY mode (NOT repair install mode)
this dumps you at a command prompt.
you can try the commands fixmbr to fix the master boot record, and fixboot to fix the boot section, (which is different from the master boot record, but that's not important right now).