How do I completely remove Ubuntu?

Solution 1:

The graphical installer will ask you what to do. Among those options is to replace your existing Ubuntu installation. I think it'll even keep your files. As always, though, it is a good idea to have backups when you're doing things like that. But I think you should figure it out if you just boot 11.10 graphical installer.

Solution 2:

If you intend to wipe the operating system and all data on the harddrive, then you should try Dban (Darik's Boot and Nuke).

Go to http://www.dban.org/download to download a bootable ISO image, burn it to a CD-R or DVD-R media, and boot from it. Follow the on-screen instructions.

Beware that this will remove all data on the harddrive!

Dban is a tool commonly used to securely erase data when decommissioning a system.