Multiple Windows Systems on an USB Flash Drive

Hello, and I want to thank everyone that can help me with this issue that I
have, and would like to overcome the challenge, that this question will bring
me, and to the one that wants to help me.

I would like to know if its possible to run off many Windows Operating
systems off a flash drive? For example, I would like to have Windows XP all
versions, and Windows Vista, all versions on the USB flash drive. How can
this be done? I don't mind if the interface is a comand line, but with a
nice menu, and which version to choose to install.

Is this possible, or am I just going crazy?

Thank you.
--
Stuserver