It's not clear what state your PC is in
I take it your computer became unusable, so you decided to reinstall XP, is that right? When you tried to install, how far did it get? Did it copy files? Did it format your hard drive? Does your computer not have a sticker with the product key? What is it on the computer that you think will give you the product key? Have you tried calling the manufacturer, giving them the serial no. of your computer and seeing if they can give you the key? Wasn't the key included with the packaging for the XP install disc? Did you make a system backup of your computer? Do you have the discs to restore to factory settings? The answer to your questions depend on knowing these things because it's not obvious to me what you think you can get from the hard disk to help you and why you think that's the best approach. For one thing it's not obvious that by attempting to reinstall XP you haven't wiped out whatever you had on the hard disk.
If you really need something to boot from to access your hard drive, you can create a Ubuntu DVD and boot to a desktop similar to the Windows one, but it isn't at all obvious at this point that this would be the most productive thing to do.