In dual booted Windows-Linux setup you can easily boot one system and use VirtualBox to boot to the second one later.
This works flawlessly for me. Whenever I start the computer I choose to boot to either to Windows 7 or to Ubuntu 12. If I later realize that I need the second operating also I start it in VirtualBox which is set up the way to use the physical disc instead of virtual ones.
However Windows 7 keeps annoying me with the message: "This copy of Windows is not genuine! You need to activate Windows again." I did it, and I did it many times. I activated them when the computer is booted to them and then after starting the same instance of Windows in VirtualBox Windows thinks that the hardware changed significantly and it wants to be activated again. But if I activate them in VirtualBox they complain everytime I boot to them normally. How can I solve this?