Unheard of program and want to install windows XP

Question

My wife bought home a new computer from her work the other day that her job had given her for free. This computer has a 2.54 processer and 512 MB of Ram, but  the only problem is that it is running a system called Ubuntu. I have never heard of this program and after visiting a few sites, I found that it is a program used in africa. I want to take this program off the computer and install windows Xp instead. How do I do this?