how do you get to dos on ubuntu

Natty is not supported in any way. It is EOL. I recommend a clean install of Precise which is LTS and supported til April 2017.

Ubuntu does not have DOS and uses BASH in the terminal. You can install Dosbox (but not in Natty as there are no sources for it anymore) and run DOS applications.

If you wanr to install Windows XP (Which only has support until April 1st 2014) and remove Ubuntu then this is NOT an Ubuntu question and should be posted on a Windows forum.

This forum only supports Ubuntu.

You should just be able to boot from the XP CD and let it install replacing everything else.

Take a look here: http://askubuntu.com/q/133533/107450

If you have any problems however try asking on

http://superuser.com/

or

http://windowsforums.org/

We don't support Windows here

You can uninstall applications, but you cannot write "uninstall an operating system", because there is no such thing as uninstalling an OS.

You can replace an operating system by using the boot CD or bootUSB stick of a different operating system (like for example Windows XP)

The boot CD of the other operating system (for example: Windows) will handle the (re)partitioning of the harddisk so you can choose how to install Windows.

So this is not an Ubuntu question, but purely a question on how to install Windows.

Why? Because Ubuntu will NOT prevent you from installing a different operating system.

We only provide support on Ubuntu related issues.