Multi-Boot Without Rebooting

i would like to know if there is a way to multi-boot a computer with only three windows xp systems in such a way as to select which system you can log into without rebooting the pc. say for instance that i am in system 1 and i want to log out and log into system two or three. does this sound like sharing a partition? can you do this with identical systems? can i use a certain software to do this? i want to effectively have a multi-boot pc with only windows xp which can switch systems without the reboot. any comments?