Hi
Do you have a Dell XP recovery CD?
if so boot up format existing C: and re install Dell XP
If not contact Dell on how to buy a recovery XP CD
(much cheaper than OEM or retail XP as they only work on Dell PC's
(two major types of Dell Recovery CD:- standard & XP Pro, then with SP1, SP2 or SP3 integrated into XP)
Or buy a Windows XP retail CD
but unless your laptop has a floppy disk drive you may have a problem if SATA hard drive controller is set to AHCI mode and you may have to change it to ATA/IDE compatible mode in the bios.
AHCI driver is included in Dell XP but OEM & Retail lack the driver which has to be supplied on a floppy disk.
Why not make your Laptop a Genuine Win 7 Home by buying a licence from Microsoft?
(unless you need the Pro for conecting to company networks)
regards
Mike Barnes