I would recommend a custom Ubuntu install using the "alternate" Ubuntu installation disc or the Ubuntu Minial CD Image. Using those installation methods you can choose to perform a command-line only installation and then build up from there. You will end up with a snappier OS with only what you want installed. It might sound daunting if you're a beginner, but you will definately learn about your machine and about Ubuntu. It's not as hard as it may seem and I do this for all of my Ubuntu installations. Here's a quick example for installing Openbox instead of XFCE:
1. Install the command-line system using the alternate or mini.iso. Once you load the disc, highlight the first option (Install Ubuntu), hit F4, and then choose the command-line option.
2. Install your packages:
3. Start Openbox:
sudo aptitude update
sudo aptitude install xserver-xorg openbox openbox-themes synaptic obconf obmenu medit pidgin claws-mail pcmanfm
This will give you a very basic, lightweight, and responsive base from which you can build up from and completely customize. If you're interested in Openbox then this is an excellent resource: Urukrama's Openbox Guide for Ubuntu. That guide will help you setup a panel, themes, etc.