How i can full format my computer using bootable cd?

If you use the Format command or use the Disk Parition utility which comes during XP Installation the Hard Disk would be formatted but by using some tools Files can be recovered.

If you want to completely remove all the files from the Hard Disk you need to do a Zero fill.

You can go to seagate.com and download the ISO version on Disk Manager software burn it on a CD and boot the system using the Created Disk Manager Disk.

You would have an option in utilities to Zero fill the disk. it will take long but would wipe the entire data beyond recovery.