|If you want to install XP again......|
If you got this computer new, it probably came with a Dell XP Re-installation CD.
That can be used the same way as a regular Windows XP disk, but only with your model.
If you don't have that, you need either a regular OEM Microsoft XP CD for the same version as on the official Microsoft label that's on the bottom of the laptop, Home or Pro, or a bootable copy of one, if you want to be able the use the Product Key on that label. A regular OEM Microsoft XP CD has "for Distribution with a new PC only." printed on it.
Do you want to install XP from scratch, which will delete everything on the C partition,
or - do you want to try running a Repair installation of XP, which will NOT delete the personal data that's already on the C partition that you have added and will attempt to repair XP ?