First, you do not format a computer, you format the hard-drive or a disk.
Second, you do not use DOS (not recommended) for XP does not use DOS but
has a DOS emulator.
Third, you do not download, you install.
If you have a retail version CD of WindowsXP, just insert the
disk and just follow the prompts for a clean install. It will automatically
format
your hard-drive and install. If you are at the "C:" prompt, you will probably
have to type D:setup and press Enter if "D" is your CD-Rom drive. If that
doesn't
work, reboot your PC with the CD in the drive, WindowsXP CD is a bootable
disk.