As far as I'm aware, and I've done this in the past thinking it was completely legal, it is fine to use a warez site to download a copy as a backup. As you say, it's the license to use the software that you are buying, not the software itself. Just be aware that to have it install correctly you will need the right version of XP - i.e. Home/Pro/Corporate SP1/2/3 - otherwise it will just say you have an invalid key. Your other option, if you're not comfortable with that, would be to give Microsoft a call. They may well, on approval of your serial number, send you a set of CDs. What about a set of OEM discs from an independent computer retailer somewhere? I'm sure if you got friendly with one and explained the situation you'd be able to borrow a set to install from.