If trying to install Ubuntu (or Xubuntu, Lubuntu, Kubuntu) 14.04 (or 14.10) on a Pentium M computer, you may get the following error:
ERROR: PAE is disabled on this Pentium M
Just restart the computer and when you come to the install menu…
Now, hit Return, and startup/installation of Ubuntu should proceed just normally.
PAE is a CPU feature that has been available on most x86-CPUs since the Pentium Pro days. Since Ubuntu 12.10, it is a required feature. Some Pentium M CPUs have the PAE feature implemented, but the processor does not announce the feature properly. Since Ubuntu 14.04 the above forcepae option is available, to allow Linux to use PAE even if the CPU officially does not support it.
This affects mostly laptops from perhaps 2000-2005. These laptops are often good computers with 1400-2000MHz CPU and 512+ MB of RAM. As Windows XP is now officially unsupported by Microsoft owners of such harware might want to install an Ubuntu flavour on the computer instead.
There have been ways to make this work with Ubuntu 12.10-13.10. I suggest, abandon those versions and hacks completely, and make a fresh install of 14.04.
I have written before about Ubuntu on Pentium M without PAE.
Migrating from Windows XP
I would personally suggest Xubuntu or Lubuntu as a replacement for Windows XP: Both should be lightweight enough for your Pentium M computer, and both are easy to use with only a Windows background. Lubuntu is most Windows-like and the lightest of them. Xubuntu is a bit heavier (and nicer), and also resembles Mac OS X a bit.
I suggest the “Try Ubuntu without Installing” option. You will have an installera available inside Ubuntu anyways, and you can confirm that most things work properly before you wipe the computer.