That is possible quite easily.. If you use seperate hard disks for each installation!
Have a look here for example: http://www.serverwatch.com/server-tutorials/using-a-physical-hard-drive-with-a-virtualbox-vm.html - or search the VirtualBox website (which is down at the moment) for these keywords.
I use it to be able to access my Linux partition when I do some Windows work, like Adobe stuff or other things that wont run that smoothly under Wine or Virtualisation of Windows in Linux.
Also, because I often access my tower remotely - and my whole Linux hard disk is encrypted, while my Windows hard disk isnt and only has non sensitive data on it. So I start up Windows (the Windows entry in GRUB is preselected) log into Teamviewer, start Virtualbox and then my Linux hard disk.
In this direction - Windows -> Virtualbox -> other HD with Linux -> it is possible quite simple, because of the superiour hardware detection, on the fly, that Linux, namely Ubuntu, supports.
In the other direction, you will end up with a mess. Windows doesnt like ever changing hardware.. It could mess up your system, doesnt start at all, or even if it does, maybe tells you that your license isnt valid anymore because too much hardware changes happend too often.
That said, you want to keep Windows as preselected boot entry in GRUB, but really watch out that you select the right entry in GRUB when it runs in Virtualbox. If you have problems with lags remotely, increase your boot timeout in GRUB to be safe.
Hope that helped you :)