firstly Ubutnu is free; you don't purchase it but you can donate if you want to. :)
...according to your information you have a "32bit" system so you should download the 32bit version of ubuntu. here's the download link http://www.ubuntu.com/download/desktop/questions?distro=desktop&bits=32&release=latest (scroll down; there's a hyperlink to the left)
try again and feed me back here ; )
...I have something more to tell you; there are two supported Ubuntu versions: 12.04 LTS and 13.10. The link above is for 13.10; I recommend it for you.
13.10 is the most up to date version but will be supported for a short time (about 5 months left); this is just because newer versions will come up so they stop supporting 13.10 to support the new one (which will be 14.04). 12.04 is not the newest version but will be supported long (several years).
put in mind that it's so easy to upgrade Ubuntu 13.10 to the next version later from the update manager; just a few clicks and you're there! Anyways it's up to you :), here's the link to 12.04 http://www.ubuntu.com/download/desktop/questions?distro=desktop&bits=32&release=lts
edit: those suggestions below are great! (the LXDE and the LXLE!)
I've never heard about LXLE but I do use LXDE!! YOU CAN INSTALL LXDE AFTER YOU INSTALL THE REGULAR UBUNTU!! Actually LXDE is ubuntu but with lighter user interface and apps. After installing ubuntu you can install the LXDE interface from the "ubuntu software center" and then you can chose which interface you want on each startup! the regular "unity" interface or the LXDE! That way you have both!! : )