|If you do decide to format the hard drive and re-install XP, then you will need the C.O.A (licence key) that was used to install windows when it was repaired. This should be recorded on the new licence sticker that your repairer should have stuck on the base unit at that time.|
If he has not done that or you are unsure whether the C.O.A. (license key) label on the base is the original dell (should say so on the label) or the new one, then there is another way of finding your Windows key.
This requires that your current XP is still runable. Find and download a utility called magical jelly bean keyfinder from:
Run it and save your licence key. You can then reformat and install a fresh copy of XP using that licence key.
☺ When everything else fails, read the instructions.