Every bloody time.
DrP – to reinstall XP and for your key to work, you’ll need to match up the version correctly (there are loads of subtly different ones). This is compounded by the fact that MS are being awkward about activation of XP – last time I did a reinstall, I had to ring them and they wouldn’t help me unless I had an original disc. They suggested at one point that I buy Windows 7 (for a ten year old computer).
Your best bet is to have a look on the website for your computer, some manufacturers offer replacement recovery discs for a nominal fee.
If it’s a regular OEM disc, I can probably build an ISO for you. If it’s branded in some way though (ie, if it says “HP” or some such on your sticker), it probably won’t work.
It’s a pain in the arse (and the Linux fanboys will be lapping this up no doubt), but it’s no wonder people pirate software when they make it so awkward to do things legitimately.