deb http://ppa.launchpad.net/lxde/ubuntu/ hardy main universe multiverse restricted, then install lxde and lxsession (sudo apt-get install lxde lxsession).
After installation, logout. Switch session to LXDE, then login again.
LXDE is really fast, but pcmanfm don't support smb:// and network:// protocols, so I just cannot switch to LXDE...
Additional, there is a source:
deb http://people.linux.org.tw/~pcman/ubuntu/ ./in LXDE's download page.