You can do a repair install. Put in the XP Disc. Let it load. From there click on install XP. (Hit Enter) Agree to the EULA (F8). Then it will detect a previous install of XP and give you the option to repair.
Yes you will have to download all of the updates again including SP2. You can use nlite to slipstream sp2 into your XP install to save you some time.