there are a few things you could try before you actually call the pc beyond repair.
Firstly if it is dell try to resore the machin as long as there is nothing on it you want, remember if you want any of the data like pictures or emails etc from it when you reinstall you loose it all. You need to take this to a store or a friend to recover before you do anything (simply remove the hard drive and recover the data before you do anything)
if your happy the drive hasnt anything and your good to try then firstly try restore, as it is dell if its relatively new try f11 or whatever it stays on the first start screen and follow the instructions.
Other method is, look a the pc for a "windows xp certificate" sticker, this has a code on it for you to use. if it has one then you are in luck, you can borrow any copy of windows xp and try to install from that, the key means you dont need one on the disc so you could get a friend to download a copy, its not illegal to do this as you already have the key! install and reboot.
If you are feeling a bit techie, you could try to repair your machine, from the sounds of things the only problem with itis that the boot record, the bit of the disc that tells it how to startup, is broken, this can be repaired using the recovery tool on the xp disc. insert it and select recovery.
Oh might mention though that all this is for nothing if the actual disc is broken or has reached the end of its life, again if you have a tech friend or another computer, if you download something like Ubuntu which is a live linux (dont panic) you just burn the disc put it in the computer and turn it on. If it starts you can see if the drive is actually still working.