Also, German, you're going to have to find drivers for your HD controller that XP can load. It's most likely a SATA controller that the XP CD doesn't have drivers for... which means you'll need a floppy disk with the drivers or you'll have to find a way to slipstream them into the XP CD.
So, even if you format the hard drive, you still won't be able to install XP, because it still won't be able to see the hard drive.