well as the first answer said running the mbr fix utility is a good answer, except you have to get an operating system up to execute the command, because as you have seen the hard drive is out of action. a couple of steps that I would do. first do you know how to start a cd up when you power on the s ystem. usally you hit the f12 key when you see the boot screen, this is the screen you see before windows starts loading, second do you have your xp installation disk. this disk has to allow you to reload xp just as the os, not like some companies send out, a complete overright when you put it in. If you have that then do the following
put the os reinstall disk in the cd drive. close door, power off system (hold down power button)
turn on power button and when you see the boot screen, tap f12 key or what ever key gets you into your alternate boot menu.
move with arrow key down to cd drive option. press enter and then press the space bar, you have about 3 seconds to do the space bar. if you miss it you will go back to the disk to load windows, if that happens hit the power button and restart.
if you get the cd started it will load a number of windows files first to memory. it will then offer you a start screen that says you can install - hit enter- or you can press R to repair. Hit R
you will then start the recovery console up, you will be asked what operating system you want, pick xp, you may be asked to provide a password to the system administrator user code. pray that you do not have one and have forgotten it. hit enter and see if it keeps going. if it does your ok, if it says wrong password, hope you can find one, or you are going to have to go to the second gate of hell.
if you do get thru, the prompt should be c:\Windows => if you get to that point, you can either type the command that the first person gave you or, you can type in chkdsk /r, this effectively will do the same plus try to fix anything else that may be corrupted on disk.
if it goes ok, your good, if it happens again in a week or two its time for a new drive.