virtualbox.org

Is there any way to boot a linux (ubuntu) I previously installed on an another harddrive from within a virtual box on windows.
until now i have to reboot the computer to switch to linux. it would be nice to have both an instant way to start linux from within windows AND the possibilty to start linux by "seriously" booting it.

this would be really great. any ideas?

You can check for some major steps on how to do this in the Windows Guest forum. There is a sticky there that allows you to run a physically installed Windows in a VM and normal. You only need to do a few steps, make sure that you have a proper MBR and you're done. Linux shouldn't need to have a special kernel or anything, but you might want to add the kernel parameter to disable SMP.