Read me for a complete guide to reinstalling Windows on a Dell.
You will need the Windows Reinstallation CD. If you don't install the same version of Windows that originally came with the system (eg, XP), you'll have to purchase a license for the new OS. If you don't have the disk for the original OS and you're in the US, you can request it here. Dell may or may not charge you for it.
If you install XP, be sure to update to Service Pack 3 (free from Microsoft) because Service Pack 2 is no longer supported. SP3 will only be supported by Microsoft until June'14.
