Lubuntu, a lightweight Ubuntu
“The objective of the lubuntu project is to create a faster, more lightweight and energy saving variant of Ubuntu by using LXDE, The Lightweight X11 Desktop Environment, as its default GUI. … Members of the lubuntu team take care of LXDE and other packages that are part of the lubuntu installation. Please join us and contribute creating a lighter, less resource hungry and more energy-efficient Linux Desktop. It is our goal to make Linux run on more cost-effective machines and MIDs besides Intel CPUs based on ARM and Mips.” (source)
I haven’t tried it, but it’s good to know about it. It may be a better choice for an old machine than Ubuntu.
Even if you don’t want to install it, visit the home page of the project, they have lots of screencasts that can be useful for the Ubuntu users too.