I have helped a friend install ubuntu on one of their custom built desktop systems. After we upgraded to ubuntu 12.04 from 9.10, my friend noticed a speed reduction in the boot process when booting the system using a pae kernel.
We brained stormed ideas of what could cause a slow boot. After which, he booted the system using a generic kernel without pae. To our surprise, the system boot speed increased. So what is pae
PAE stands for, Physical Address Extension, a technology used by 32bit sytems to access 64GB of memory. This is not needed for 64bit systems.
If you have a cpu does not support PAE, you can still use 12.04 LTS. Newer versions of ubuntu will have pae enable by default which could be a problem on non-pae cpus. That being the case, you can use Lubuntu or Xubuntu that still ship with non-pae kernels.
This link tells more about PAE
http://askubuntu.com/questions/19389/whats-the-meaning-of-pae-at-the-end-of-kernel-version
If you have an older computer that is not pae enabled, and you want to use the latest version of ubuntu, this link will help accomplish that.
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