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
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.