Well, when you press the 'd' key to delete the partition it means it will delete all the information in your hard drive and therefore format your computer. If you've never installed an operating system I would recommend you to have a friend or someone who has done it before helping you out. It's very easy though, just put your XP CD in your cdrom, restart the computer and it will start from the CD, it will start recognizing all the hardware, then it will guide you through the installation, like I said, at some point in one of the first screens it will ask you if you want to use that partition or not, if not, hit 'd', deleting the partition, then you have to create a new one, all the instrucions are given through the installation.