If you have old hardware try Xfce desktop system.
Xfce is a lightweight desktop environment for various *NIX systems. Designed for productivity, it loads and executes applications fast, while conserving system resources.
Xfce 4.4 embodies the traditional UNIX philosophy of modularity and re-usability. It consists of a number of components that together provide the full functionality of the desktop environment. They are packaged separately and you can pick and choose from the available packages to create the best personal working environment.
Mayank Sharma has posted a nice review:
For years, the lightweight Xfce has been a popular desktop environment for Linux distributions running on older hardware, thanks to its lower demand on resources as compared to KDE and GNOME; it's an ideal desktop for machines with less than 256MB of memory.
You can simply install xfce4 under Debian or Ubuntu Linux with following command:
$ sudo apt-get install xfce4