Some mad person apparantly got Windows 95 running on a PSP. This is done using the bochs x86 emulator which emulates various CPU's. Using this method various other OSes could be run as well in theory. There are some "slight" problems: It runs extremely slow and the controls aren't too great either. The latter could possibly be fixed using a USB keyboard. So basically it's just a demonstration of the possibilities, though if you feel like doing it yourself (or want to find out how slow it is) look here.