Lightweight X11 Desktop Environment or LXDE is a deasktop environment for netbooks or desktop having old hardwares. It is the default desktop environment of Lubuntu. You can use it on other Unixes like FreeBSD. It consume less resources like ram, CPU, and power.

If you are using Ubuntu/debain/theirderivatives type this:
apt-get install lxde
For rpm based fedora, mandrive type this:
yum install lxde

If earlier you don't have any desktop environment type this:
echo "exec startlxde" >> ~/.xinitrc
Then type this:

If you already have desktop environment you will get it as options from login window of gdm or kdm.

At first when you start it will give you a warning that you don't have an icon theme set. You will have to select item theme from application menu --> Preferences --> Appearance.
Not all options are set through that you will have to right buttons to see others.