You realize this means deleting all of your files, right?
Make a boot disk that has fdisk on it. You can use a windows 98 disk from www.bootdisk.com
The file you download will probably be an exe file, so put a blank floppy into the drive and double click the exe. It will automatically put the files on the floppy disk. Make sure that fdisk is on there. Now restart the computer with the floppy disk in. At the A:\ prompt, type fdisk. Format the hard drive and create a primary partition. When you're finished with fdisk, take out the floppy, put in your Windows XP CD, and restart the computer.