Tone,
If you are going to buy a copy of XP from Ebay. You need to make sure it had not been registered before. I have sold a few on the net from PC's that came with XP pro preloaded but were never going to be used as windows boxes. As soon as they were purchased I put Linux on them. There fore the CoA was never registered with Microsoft. This practise in not illegal.
Make sure, if you decide to buy, that you dont buy a Dell copy. These will only work on dell machines. I have sold HP copies in the past and know that they are FULL copies of XP Pro.
Hope this helps.
Mike