Exactly as Chris said. For example with XPe you can add Windows Media, .NET Framework, Explorer Shell but with Win PE that's almost impossible (some of it is possible but you will have problems later on). XPe also does not have the same limitations as Windows PE such as rebooting after 24 hours and the 6 process limit. It's fully customizable and more personal, becuase you can add your own drivers, with gives the user more choices.