Hi AnniMc,
If you are making an exact image backup then the OS should just transfer to the new harddrive. Otherwise, what you need is to download a copy of WinXP OEM (the OEM is important) from a torrent site. While that is going on, download 'Keyfinder' from
http://www.magicaljellybean.com/. This program will extract your license key from your windows installation.
Install the OEM windows XP on the new harddrive using this serial key. It has to be the OEM version of XP otherwise the serial key wont work.
This isn't illegal as you have a legitimate serial key.