Virtualbox has so-called 'seamless' mode, in which the X11 windows from VM running on Windows host appear as separate windows in the Windows desktop. However, they only appear separate; they don't have separate panel button and they are not directly reachable via alt+tab. One needs to switch to VM first, then use keyboard shortcut set up in VM to access them.
If you alt-tab it to the vm, you have to alt tab again to switch between linux programs.
Usually that's not a problem, because I only have emacs and/or a terminal session running. With the control key on the right, you switch back to your host os.
To switch to a windows program from linux you simply press RCtrl+Alt+Tab.
Virtual desktops in Win 10 are very handy while using a VM. Ctrl+Win+d opens a new virtual desktop. Ctrl+Win+Arrow Keys switches between them.
Tell me more. Which one?