HOW TO INSTALL WIN XP/7 ON USB ?

Is there any way to install win xp/7 on usb/pendrive. I m not talking about Bart Pe. I mean to say that install full windows on pen drive as we install on hdd, boot from pendrive & run everything as normal windows installation. I dont mind using even 16 or 32 gb pen drive. I dont want any hdd in my system.