How to Install Windows 7 within your existing OS
 

Trying out Windows 7 isn’t quite as difficult as you may think, as you don’t need to wipe or dual boot your existing Vista/XP installation to try out the new Microsoft operating system. It is entirely possible to run Windows 7 from within your existing Windows installation, at the cost of reduced performance. All of this is thanks to virtualisation technologies which allow you to run a computer within a computer.

There is a downside to this, as a virtualised PC normally uses emulated hardware – so the graphics card used to power Windows 7 will be nowhere near as good as a modern gaming card. This means you won’t be able to play recent games within the virtualised installation, but it is more than adequate for testing and playing with the new OS.

http://www.w7forums.com/run-windows-...lbox-t107.html

Do note that Aero doesn't run in VM afaik

Apparently aero will run under parallels, though you probably need OSX to run that :P

Neither in VirtualBox, plus the text-based part before booting is extremely slow (it updates the screen line per line at a speed that would be considered average in 1980)