run a second / third windows or linux installation inside a window in your normal os.
boot an iso in a window.
run windows 7 , xp , vista inside a linux window,
run linux inside a "windows 7" window,
run windows 7 in windows 7 ............................

http://www.virtualbox.org/wiki/VirtualBox

There are many portable versions find with google:
moderation: link of google search results removed. Please read Rules for applications' submission

Please check http://www.vbox.me/

It is unofficial.