Which computer are you using now? What made you decide that the Dell needs a COMPLETE overhaul? The 98 computer was using FAT (file allocation table) 32 for the hard drive. Windows XP uses NTFS for the hard drive. Are you switching hard drives? or maybe you weren't given all the right info.
Spybot-Search and Destroy is a very good program for cleaning a computer, it's free (not even registration), just put the name in the search bar and press "enter". Microsoft Vista is due out early next year, if you will be uploading anything to office networks, consider if there might be a compatibility problem with their O/S or word processor.