Rite, if you buy a new hardrive and you have a brought computer with xp back up, you will be able to install the new back up onto your new harddrive, if you have 2 computers and 1 with a backup of xp you will not be able to put them onto the other computer which came with no xp backup.
Get windows xp copied from www.mybittorrent.com
or torrentspy.com you will have to download BITTORRENT
which can be found on, google.com or yahoo.com, download that and then leave it, once you have done that go to www.torrentspy.com
and search for WINDOWS XP PRO or Home edition, download it, once download it download NERO from www.nero.com
go for the trial, once doing that, insert a blank copy CD or DVD R depending on the size of the XP file, after you have inserted the disk, open NERO then click on RIGHT A FILE TO IMAGE, (ISO) then ADD your windows Xp which your downloaded then wait for it to but it onto disk.
Once doing all that restart your computer with the WINDOWS CD which you have just done, then install the cd from there.
If your pc does not boot with the cd, go to BIOS by pressing DEL or F8 then look for ADVANCED options then it will have a list of all your drives E.G (FLOPPY, DVD RW) once you have seen that, go to the first dive you c and but it as which ever your bootdisk is inserted to.
Sorry but that a fast typed GTG cya