I’m sorry I might be wrong.
I don’t remember clearly. You could make it even if you can’t boot your computer with a bootable CD.
Run Windows XP installation from your hard disk. Check if you have an option of a clean installation. After this clean installation, you can delete the previously installed Windows.
Or, if you want to keep all your existing settings, you may choose upgrading from your current OS (Windows 98 or Windows 2000, I guess) to Windows XP. This upgrading method is known as not stable as a clean installation.