NTLDR is missing

ive read the articles on this but cant find the solution.
my problem,the pc crashed so i put the HDD on another pc
and formatted it,then put it back and reloaded xp.it gets
halfway in installing then reboots and gives the ntldr is
missing error.how do i fix this.
i also put a new HDD with xp installed already and booted
up and sometimes it hangs on the POST and sometimes gives
me the ntldr message.
pls help me this is the office pc and it needs to be
fixed soon.
thanks