Check the BIOS settings that it reads CD's on bootup.

Insert your XP CD into the drive then turn the computer on or reboot. Follow instructions to format and install XP.

You haven't said why you want to reformat the drive. I assume you want to re-install Windows XP.

Fuller guidance for a range of situations can be found here;
http://reviews.cnet.com/5208-6130-0.html?forumID=7&threadID=66200&messageID=785595

Mark