This kernel requires the following features not present on the CPU: pae Unable to boot - please use a kernel appropriate for your CPU.
So, this means that if you want to install Ubuntu v12 and up on hardware with older processors, you may not be able to do so from the downloaded ISO images. Canonical Ltd (the developer of Ubuntu) warned that they would drop support for non-physical address extension (PAE) processors in 2011, well before v12.04 was released.
According to this ServerFault.com forum post, “PAE is provided by Intel Pentium Pro (and above) CPUs - including all later Pentium-series processors except the 400 MHz bus versions of the Pentium M, as well as by other processors such as the AMD Athlon and later AMD processor models with similar or more advanced versions of the same architecture.”
You can still install Ubuntu 12.04 LTS on older hardware, but you need to follow one of the documented workarounds to do so. Take a look at the Ubuntu forums for guidance: http://askubuntu.com/questions/117744/how-can-i-install-12-04-on-a-non-pae-cpu-error-kernel-requires-features-not-p.
What I did to work around this issue is first install Ubuntu v11.10, then perform the OS upgrade from the software updater. It took several hours to complete the upgrade, but at least I now have an up to date installation of Ubuntu. It may be a good idea to keep a copy of Ubuntu 10.04 LTS or 11.10 in your software library as a backup…