|Yes you will have to get Windows working on it again - no sensible person is going to buy a second-hand PC unless they can see it working properly.|
The copy of Windows that was pre-installed on that laptop at the factory is tied to that laptop for the rest of it's life and should by rights remain installed on it when you sell it.
There are two possible reasons I can think of why Windows XP Setup cannot now find the hard drive:
1. Since wiping all data off it, if it's a SATA hard drive, the driver for the hard disk SATA controller is no longer present. Since Windows XP does not support SATA disks natively, you need to re-install the SATA driver during XP Setup by pressing F6 and telling XP Setup where the driver is on a pre-prepared floppy disk. See this: http://www.alexnolan.net/articles/i...
2. If it's a standard IDE-PATA disk (not SATA) the above does not apply because XP Setup doesn't need a third-party driver for standard IDE-PATA disks. In this case if you get "No disk found", the disk itself may have failed or is not connected up properly (unlikely on a laptop). A failed disk will have to replaced by a new one.