give me full details plz

If you wan to do a clean still get a partition manager software and wipe one partition off, that is if you have your HD partitioned.

Once insert the OS CD you want to install into the drive, reboot pc, and boot from CD. Most BIOS will auto detect the CD and ask if you wanna boot it, if you can't go to your BIOS setup and tell it to boot from CD first.

Then just follow instructions, if you're installing XP tell it to do a clean install and it will reformat your HD and install it, your other OS should be gone...