You want to re-format and reinstall XP, right? in that case all you need to do is make sure that your computer can boot from a CD drive:
put the XP cd in the drive and reboot
choose to boot from the cd rather than the harddrive - screen should say "searching for boot record on CD-ROM" and then "Push entert to boot from CD"
the WinXP install should fire up; after a while it will ask you what you want to do. Choose a new install, even if it detects XP as already on the hard drive.
After asking you to confirm a new install, it should pop up a screen asking where to install it, and giving you a choice of harddrives. From here you can choose to add/delete partitions, format drives, or install XP. You can use this screen to re-format the harddrive.
On the other hand, if you're trying to get rid of XP and go back to windows '98 (you mentioned using DOS) just find a place to download a '98 boot disc. Start the computer with this and use the fdisk command to format the harddrive.
Hope this helps.
Nate