This is just an idea but maybe each copy of xp is identical it is only the licence code on the sticker on the machine that makes it different. Therefore if you can borrow somebodies full xp disc but use your licence key to allow it to work maybe that could solve the problem. I think when you pay up for your computer or your copy of xp you are really paying for the licence for the program which you've already done when you bought your pc.
If you were a company and wanted xp on 100 computers you'd probably get 100 licences and one disc, is that correct?