XP HOME REINSTALLATION AFTER FORMAT

Hello,
I LOST MY RECOVERY CD FOR WINDOWS XP HOME. MY LAPTOP HAS DIVIDED TO PARTAS C AND D . C HAS ONLY 15 GB AND D IS 25 GB. C IS NOT ENOUGH NOW WITH THE UPDATED PROGRAMSAND ALMOST FULL. I WANT TO MAKE FORMAT TO LAPTOPBUT HOW CAN I REINSTALLATION MY XP HOME.I HAVE GENUINE KEY FOR MY XP HOME BUT WHEN I FORMAT MY WINDOWS AND I386 AND WINNT32EXE WILL CLEAN AND HOW CAN I DO THAT. I COPIED WINDOWS TO EXTERNAL HARD DISC HOW CAN I DO THAT. PLEASE EXPLAINE TO ME CLEARLY. THANKS.