|New hard drives have no data on them.|
The hard drive must have an operating system installed on it in order for the bios to detect the hard drive as bootable.
Most Dell computers that came with XP on them came with a Dell XP Re-installation CD or similar and one other disk with at least Drivers on it when they were new.
If you have the Dell XP Re-installation CD or similar, you just boot the computer from that disk, install XP, then when Setup is finished, if you have the other Drivers disk you install the drivers for your system.
Use the Product Key that is on the official label on the outside of the computer case when XP's Setup asks for it.
The bios Setup Boot Order or similar settings must be set so that you can boot from a CD.
You will have to change a bios setting if you have a SATA hard drive if the XP CD does not detect it.
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