Normal usage
The minimum specification for CPU is Pentium 4 or Pentium M or AMD K8.
Older processors are too slow and AMD K7 has problems with flash video.
Pentium M and Celeron M
Community 're-spins' based on 12.04 LTS (the original, before the point releases) can have a non-pae kernel and work out of the box. Examples: Bento, Bodhi, LXLE with support until April 2017. Notice that Lubuntu 12.04 has passed end of life.
Systems based on Ubuntu 12.04.5 LTS 'Precise' can be installed in Pentium M and Celeron M computers using fake-PAE. This can be done using the One Button Installer or Lubuntu-fake-PAE.
Lubuntu 14.04 LTS 'Trusty' and newer versions can use the boot option forcepae and can be installed using the standard installers.
After selecting language you arrive at the main menu of the installer. Click on F6
At the boot menu screen the options are
Try Lubuntu without installing (in the desktop installer but not in the alternate installer]
Install
...
With the Install choice high-lighted press F6. (This option needs less RAM than installing from 'Try Lubuntu')
A menu with a number of options appears. The option 'forcepae' is not there, so press Escape to close the list.
Now a string of options is visible, often with 'quiet' or 'quiet splash --' at the end. Add 'forcepae' to the string before and after the two dashes.
... quiet splash forcepae -- forcepae
Press return, and the installation begins.
This link describes a new way to use a boot option. Sometimes it must be entered twice, before and after the delimiter "-- ". We find it in Ubuntu Server 14.04.2 and newer versions, and in Ubuntu mini.iso version 15.04 and newer versions.
In the netboot method, also known as PXE booting, for 14.04 the 'Advanced options' menu item allows a 'Command-line expert install' which will find the right kernel. Then once the bare system is installed and booted, either the metapackage lubuntu-desktop or lubuntu-core can be installed. The checksums and OpenPGP signatures for the netboot downloads can be found in the directories above the actual downloads. Note that 'forcepae' method above may not work with netbooting, PXE, but will work with net install, mini.iso.