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.
u gotta go to bios and set ur priority boot device to cd and from there restart ur com..
when it restarts... they will ask u whether u want to boot from ur cd (make sure ur window xp cd is inside).
follow the instruction after it loads, u will want to install a windows xp and when they ask u should delete the partition and from there create a new one.
when it is finished, well, u have formatted ur C:\!