I use Windows 7. I also run Ubuntu 12.04 in VirtualBox. The other day I wanted to try Lubuntu. Is there a way to dual-boot Ubuntu 12.04 and Lubuntu in VirtualBox?
If you just want to test the Desktop Environment of Lubuntu (LXDE) you can follow this how-to and get an lubuntu desktop on top of your current Ubuntu installation.
In this way you don't have the need of a second operating system. If this is not enough for your tests I'd suggest to install a second Virtual machine is much more easier than having one with dual boot.
With VirtualBox you can have any number of virtual machines, each with its own OS. There's absolutely no need to complicate things with dual boot unless you want to play with configuring dual boot.
Keeping OSes separate makes it much easier to play with different distros etc. - you can just delete the whole machine when you're done with it.
As a bonus, if you have enough RAM, you can run multiple virtual machines at the same time, no need to reboot from one to another.