I disagree. You are not re-installing the entire OS. I think it will only access whatever files it needs to make the repair, assuming it can be repaired at all. So, any authentic XP Pro disc will work. If you have to, buy a copy from a reputable ebay merchant, making sure you get the license key, fix the problem, and re-sell it on ebay when you are done.
If it is a Dell, the machine is already licensed by Microsoft. Just go to a small computer shop, they can "fix" it for you for $30~$50.